Chthonerpeton indistinctum (Reinhardt & Lütken 1862)

It is a typhlonectid with a body length between 99 and 530 mm and 70 to 80 incomplete primary body annuli. Secondary annulli are absent, and the skin is smooth. The dorsum is bright black, the venter is black-gray, and both are covered with minute white spots. In addition, The tentacle and the eye are surrounded by a whitish area which can or cannot be continuous. The disc area surrounding the vent is cream-white. The nostrils are dorsolateral and closer to the snout than to the eyes, and the tentacle extrudes midway between the eye and the snout. The skull is similar to that ot other typhlonectids, but with an open tentacular groove that crosses the maxillopalatine. The quadrate and pterygoid are completely fused.

C. indistinctum is found in temperate South America, in the Rio de la Plata basin and in drainages along the adjacent coasts.

Diagnosis A species having a total length from 99 to 530 mm. The skin is smooth; primary annuli are incomplete at the dorsal and ventral midlines and their number shows lower varation (extremes 70-81 for adults); secondary annuli are absent. Head dorso-ventrally compressed; prominent snout projecting beyond the mouth; nostrils oval, directed dorsolaterally and clearly closer to the snow than the eyes. Tentacle in a white area that projects posteriorly to the eye and is in contact with it or not. The bead is wider posteriorly and the distance between the eyes is greater than that between the nostrils. Body width is greatest at the middle of the body, followed by the width at the neck and is smallest at the terminal tip.

Found in in the Rio de la Plata basin and in drainages along the adjacent coasts. In Argentina the species occurs in the Buenos Aires province of the Atlantic coast, and at localities adjacent to the Rio Paraná in Entre Rios, Santa Fé,Corrientes, and Chaco provinces. In Brasil most of the records are from Rio Grande do Sul state (Atlantic coast, Lagoa dos Patos - Rio Jacui basin), and a few records are from Paraná and Santa Catarina states. In Paraguay from a only locality in Misiones department. In Uruguay the species has been collected in the Rio de la Plata and on the Atlantic Ocean coast, with an isolated record from the mainland.

Description of the lectotype The eye is rather large, very distinct, the distance between eyes (4.9 mm) greater than the snout length from the front level of the eyes to the tip; the snout projecting 2.2 mm beyond mouth. Nostrils slightly elongated ovals, not or scarcely visible from directly above head. The tentacle emerges a little below the direct line between eye and nostril, closer to the nostril (1.5 mm) than to the eye (2.1 mm). The two collars are rather poorly delineated by the three nuchal grooves. The latter are moderately distinct laterally but very dim dorsally. The two collars are fused together above, the second is joined to the first primary above and below. There are no transverse grooves on the collars. (A vague line is indicated on one collar, but this would appear to be merely a wrinkle.) Following the collars there are 74 primary annuli and no secondaries, the body ending in an unsegmented "shield" about 9 mm long, on the underside of which is the circular vent; it is surrounded by a series of eleven elongated denticles, the area not depressed (normally depressed?). No anal glands are distinguishable. Terminus of the body is rather narrowed, the diameter at vent much less than the width of the body (6.4 mm). There are no scales. The folds are narrowly incomplete above and below throughout the body (an occasional annulus may appear to be complete on the venter). Dentition: The teeth are in four series, the splenial being much reduced. Premaxillary-maxillary teeth, 16-1-17; prevomeropalatine, 15-1-15; dentary, 13-14; splenial, 2-2. The dentaries are somewhat larger than the maxillaries, which in turn are larger than the other teeth. The choanae are large (with valves), the transverse diameter of one being a little less than the distance between them. The tongue has been largely destroyed. Measurements in mm: Total length, 281; body width at vent, 6.4; width of body (middle), 11; width of head (greatest), 8.3; tip of snout to first nuchal groove (lateral measurement), 10; to second, 14.2; to third, 19; width in length, about 25.5 times. Variation There is a slight increase in the number of teeth in the various series from youth to old age. Perhaps some of the dental variation is actually the result of failure to count correctly the temporarily absent teeth, although an effort was made to do so. It is possibly due to geographic variation. The relationship of length to body width varies markedly, especially in gravid females. When not gravid they may differ but little in proportions from equal-sized males. The position of the tentacular opening with relation to the eye and the nostril shows little or no variation in adult specimens, the nostril being definitely closer to it than the eye. In the very young, the tentacle, when it first appears, may be closer to the eye but gradually moves farther from the eye. Thus at one time it may be equidistant from them. One point should be stressed: the measurement should be made from the pupil of the eye rather than from the edge of the larger white spot. Failure to do this would of course change the proportions. The two nuchal collars are usually indistinct in this species, as are the three nuchal grooves that ordinarily delineate them. Two small anal glands are evident externally on the anal "disc" of many if not all of the males. I am not wholly certain that these glands are invariably absent in females, although the glands are not visible in females examined. Variation in the number of annuli posterior to the two collars is not great (70-81).

LC. Least Concern.

Chthonerpeton indistinctum is a species of caecilian in the family Typhlonectidae. It is found in northeastern Argentina, Paraguay, Uruguay, and southeastern Brazil. The common name Argentine caecilian has been coined for it.