Drawing × 3.5. of type specimen of Micrerpeton caudatum Moodie, from the Coal Measures of Mazon Creek, Illinois, showing skeletal elements, form of head and tail, the lateral-line sense-organs, banded color-markings, and ventral scutellæ. On the edges of the tail impression are indications that in life the tail had a thin fold of skin above and below the fleshy portion, much as in the larvæ of Amblystoma at the present day.
The remains here described represent a small, salamander-like form, and they are among the earliest geological evidence of the group, which, without doubt, gave rise to the modern salamanders. The parts preserved in the specimen are: the complete outline of the head with most of the cranial elements clearly distinguishable, as well as the black pigment of the choroid; the entire vertebral column, including pits in the tail region, where the vertebræ were without doubt entirely cartilaginous; parts of the pectoral girdle; the ilium; the left humerus; the ventral scutellation; the ribs of one side of the body and indications of ribs on the other; portions of both hind limbs; and a complete impression of the fleshy tail. On this impression of the tail are preserved small, horny scales, transverse color-markings, and the distinct impressions of the lateral-line system.
The bones of Micrerpeton caudatum, as in so many of the fossils from this locality, have been replaced by a white, friable mineral which is probably kaolin. The animal is preserved on its back and it is thus illustrated from the ventral side. The entire length of the animal is only 49 mm., of which the tail occupies nearly half.
Fig. 13. Restoration of Micrerpeton caudatum, a branchiosaur from the Coal Measures of Illinois. × 2.
The head has much the same shape as in the species of Branchiosaurus described and figured by Fritsch ([251]), Credner ([181]), and Thevenin ([568]). The eyes occupy relatively the same position as in that genus. The orbits are very large and broadly oval. Within the borders of the rim the stone is blackened as though by the black pigment of the iris, such as Cope has described in Amphibamus. Under a rather high power of magnification the cranial bones are seen to be represented by mere flakes of white mineral matter. The sutures separating the cranial elements are distinctly preserved on the main half of the nodule.