102 ON THE STRUCTURES AND DISTRIBUTION OF THE 



full parallelism between the Raniform and Arcif'erous Anura.* It is carried out be- 

 tween the Characini and a group of remaining Physostomous fishes perhaps not yet 

 well defined :t it is exhibited between the orders Diptera and Hymenoptera among 

 insects. None of these comparisons can be allowed, of course, without the most 

 searching anatomical, histological, and erabryological analysis. ' 



This heterology is what Swainson and others called " analogy," as distinguished 

 from affinity. It generally relates genera of difierent zoological regions. Mimetic 

 analogy, on the contrary, relates genera of the same region ; it is a superficial 

 imitation which has occurred to critical biologists, and is of much interest, though as 

 yet but little investigated. It has as yet been observed in external characters only, 

 but occurs in internal also ; it has been accounted for in the first case by the supposed 

 immunity from enemies, arising from resemblance to well defended tj^pes. No such 

 explanation will, however, answer in the latter case. I believe such coincidences 

 express merely the developmental type common to many heterologous .series of a 

 given Zoological " Region." This will be alluded to a few pages later. 



To return from this digression. 



Three principal types of Urodela may be discovered in tlieir skeletal arrange- 

 ments ; viz. : 



TRACHYSTOMATA (Mueller). 



0. maxillaria wanting: nasalia embraced by spine of premaxillaria : prefrontalia 

 wanting; palatina wanting; pterygoidea wanting; orbitosphenoids large, anterior, 

 forming part of palate ; mandible with condyle, without teeth on ttie dentale. Cera- 

 tohyals, first two distinct. 



With branchise, biconcave vertebras, and cartilaginous carpus, as characters of less 

 intrinsic value. — 



Fam. S i r e n i d a e . Genus Siren. 



PROTEIDA (Mueller). 



0. maxillare and prefrontalia wanting; palatinum and pterygoideum present, con- 

 tinuous ; nasalia wanting ; orbitosphenoid elongate, not forming part of palate ; man- 

 dible with teeth on the dentale. Ceratohyals, first two connate. 



Subordinate features : branchia?, biconcave vertebra?, and cartilaginous carpus and 

 tarsus. — 



Fam. Proteidae; Proteus and Necturus. 



CADUCIBRANCHIATA.J 

 0. maxillare present; prefrontal present, (with one exception); premaxillaria 

 embraced by nasalia; palatina present, not approximated to usually present ptery- 

 goidea; orbitosphenoid large, not reaching palatal surface; mandible with teeth on 

 the dentale. 



* Proc. Acad. Nat. Sci., 1864, 181. 



t Gill's Eventognathi and Nematogiiathi being of course omitted, 



J This name, though not strictly applicable to Siredons included in the suborder, m.ay be retained. 



