100 ON THE STRUCTURES AND DISTRIBUTION OF THE 



continuous palato-pterygoid arch, in close contact with the parasphenoid. The two 

 ceratohyals are confluent, the posterior is present, and there are but three superior 

 hyoid arches. These structures coexist in the larva of Spelerpes sal mono a, at 

 a period when it has attained double the size of that of Arablystoraa punctata m, 

 and equal to that of its adult. The hyoid elements only I have not yet seen as fully 

 ossified in Spelerpes larva, as in the Necturus. The relation then between Necturus 

 and Spelerpes is probably the same as that between Siredon and Amblystoma, and 

 the same (as I have pointed out, Proc. Acad. Phil. 1862, p. 66) as that between Pro- 

 tonopsis and Megalobatrachus, in respect to the pharyngeal fissures and hyoid 

 pleurapophyses. 



Dr. Gray, after Prof Baird, has very judiciously excluded the Siredons from the 

 Perennibranchiates proper, though Miiller retained them as the type of his family 

 Acholotida. They undoubtedly belong to the group Caducibranchiata, and family 

 AmblystomidiB. A i^riori, therefore, Necturus might be placed among the Plethodon- 

 tida3 of the same great series, but its structure, so far as observations have been made, 

 is permanent. It also represents a far less advanced larval bony structure than 

 Siredon, but so do the adult Plethodontidaj in most respects than the Amblystomas. 

 The incomplete coincidence of advance in different organs may be thus illustrated : 



Amblystomid^ . Plethodontid^ . 



Superior. Inferior. 



Carpus and tarsus osseous. Carpus and tarsus cartilaginous. 



Premaxillary fontanelle closed. " Premaxillary fontanelle open. 



Inferior. Superior. 



0. pterygoideum persistent. 0. pterygoideum obliterated. 



The inferiority of some Plethodontida? is seen in the non-distinction of the digits 

 (Geotriton), the thinness of the ossification of the parietal membrane bones (Batra- 

 choseps), and in Spelerpes 1 i n e o 1 u s m, from Vera Cruz, the persistence of the 

 membranous cranium by the limitation of the parietal bones to two small oval lateral 

 scales, and the wide divarication of the posterior extremities of the frontals. 



We may then conclude that developmental features are thoroughly constant in most 

 types, but in some one or more cases in many, some are known and the others will 

 probably be found, to illustrate the law of variation above laid down. We are then 

 brought face to face with phenomena of modification of species, which do not come to 

 view in the irregular aspect of a theoi'y of " descent with modification " by a fortuitous 

 " natural selection," which could affect only a certain class of structures. We want a 

 theory which will explain the times and causes of the stability of types consisting 

 of co-existent structures, and the instability of such co-existences; just as the com- 

 pounds of the elements have their conditions of stability and instability; the .elements, 

 of stable or unstable union. We want to ascertain that law of harmony by which the 

 coincidences of structures have been varied by their reproduction being shifted from 

 stage to stage of individual development, till the present liuintu are the result. 



