NATURAL SCIENCES OF PHILADELPHIA. 258 



stage of Odax, while those with distinct teeth are the same in this point as 

 the embryos of the highest — Odax, etc. I venture to predict that here will be 

 found a long series of exact parallelism, in which the different genera, resting 

 exclusively on these dental characters, will be found to be identical generically 

 with the various stages of the successively most advanced. 



11. Professor Agassiz states that the absence of ventral fins is characteristic 

 of an embryonic condition of the Cyprinodont fishes. The genus Orestiag 

 does not progress beyond this stage in this one point. Probably the genus 

 will be found which will only differ from Orestias in the presence of ventral 

 fins. If so, Orestias will be identical with an imperfect stage of that genus, 

 if, as will probably be the case, the fins appear in the latter, after other struc- 

 tures are fully completed. 



yj. Parallelism in Higher Groups. 



It is not to be anticipated that the series of genera exhibiting exact parallel- 

 ism can embrace many such terms, since comparatively few stages in the de- 

 velopmental condition of the same part in the highest, would bring us back to 

 a larval condition, which, as far as we yet know, has no exact parallel among 

 existing genera. But it is to be believed that the lowest terms of a number 

 of the most nearly allied of such series, do of themselves form another series 

 of exact parallelisms. 



Thus exact parallelism between existing genera of mammals ceases with all 

 characters which are larval or foetal only prior to the assumption of the adult 

 dentition, since among the higher mammalia at least we know of no genus 

 which, however similar to undeveloped stages of the higher, never loses the 

 milk dentition. It is nevertheless an important fact that, among smooth 

 brained mammals, or many of them, but one tooth of the second series ap- 

 pears; and inasmuch as smooth brained forms of the higher orders have be- 

 come extinct, it is not too much to anticipate that a type of permanent milk 

 dentition will be found among the extinct forms of the same high orders. 



As an example of exact parallelism in series of series, I select the follow- 

 ing : 



1. in the Batrachian family CystignathidEe there are six groups or sets of 

 genera. In the highest of these we have an ossified cranium and xiphisternum 

 — i. e. in the Cystignathi; in the Pleurodemae the cranium is not ossified, thus 

 representing the Cystignathi while incomplete ; in the Criniaj the xiphister- 

 num is cartilaginous, as well as the fronto-parietal region, being an equiva- 

 lent of a still lower stage of the Cystignathi. From this simplest type we can 

 find a rising series by a different combination of characters ; thus the Cera- 

 tophydes add an osseous cranium to the incomplete xiphisternum, while two 

 succeeding groups diverge from each other at the start, the Pseudes loosening 

 the outer metatarsus in their development to maturity, while the Hylodes add 

 by degrees a cross-limb to the last phalange. The Ceratophrydes and Criniae 

 are stages in the development of these, but neither one of them is a step in 

 the development of the other. They are measured by adaptive characters 

 purely. 



2. The whole suborder of the Anurous Batrachia, to which the above family 

 belongs, the Arcifera, differs from the suborder Raniformia by a character 

 which distinguishes a primary stage of growth of the latter from its fully de- 

 veloped form. That is, the Raniformia present, at one period of their devel- 

 opment, a pair of parallel or over-lapping curved cartilages, connecting the 

 the procoracoidand coracoid bones, which subsequently unite and become a 

 single, slender median, scarcely visible rod, while the bones named expand 

 and meet. The first condition is the permanent and sole systematic character 

 of the Arcifera.* 



* This may be readily understood by comparing my monograph of the Arcifera, Joiun. 

 Ac. Nat. Sci. Phil., 1866, with Duges work, or Gegenbaur & Parker's memoirs on the shoul- 

 der girdle. 



1868.] 



