ON THE ORIGm OF GENERA. 49 



5. When the larvae of certain salamanders (Spelerpes) possess 

 branchiae, they also lack one digit of the hind foot, also the max- 

 illary, nasal, and prefrontal bones, and exhibit a broad continuous 

 palatopterygoid arch, in close contact with the parasphenoid. 

 The prootic is separated from the exoccipital by a membranous 

 space, and the exoccipitals themselves are not yet united above 

 the foramen magnum. There is at the same time a series of 

 splenial teeth. Both ceratohyals are confluent, the posterior is 

 present, and there are but three superior hyoid arches. After 

 they lose the branchiae, the hinder foot, which has four toes only 

 for a time, gradually adds another at first rudimental digit, in the 

 Mexican species ; in most North American species the fifth digit 

 appears at an early larval stage. Five digits are finally present in 

 all Spelerpes. 



We have thus four combinations of the above characters, at 

 different periods of the life history of certain (but not of all) of 

 the species of Spelerpes. There exist four permanent series of 

 species or genera, equivalent to these stages. The well-known 

 ^^perennibranchiate" J^ecturus is nearly identical with the first, 

 Batrachoseps with the second, the half-toed Spelerpes with the 

 third, and the typical Spelerpes is the last. 



In one character of generic value only do I find that Necturus 

 differs from the early larval Spelerpes. It closes the premaxillary 

 fontanelle with which it commences, by an approximation of the 

 premaxillary spines, but not by a sutural union, as takes place in 

 Amblystoma. It thus, in this one point, advances a stage beyond 

 the condition to which Spelerpes attains, though it may be a 

 question whether such a closure without union should not be 

 classed among the specific characters by which iV. maculatus dif- 

 fers from the young of the various Spelerpes, as they do from 

 each other. Characters of the latter kind are the following : in 

 N. maculatus the frontals are more deeply emarginate behind ; it 

 has little or no ala on the inferior keel of the caudal vertebrae, 

 which is prominent in Spelerpes larvae. 



It may be that the parallelism in the case of Spelerpes is inex- 

 act by one character, and that a strictly developmental one ; or 

 it may be regarded otherwise.* 



6. It is well known that the Cervidae of the Old World devel- 



* Necturus differs from these larvas by another and more important character, 



viz., the presence of the os intercalare. 

 4 



