MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 321 



Chelonians, Lacertians, Ophidians, but not in the common Batrachia, e.g. 

 Rana, Bufo. In osseous Fishes they are large and well-developed. 

 4. The superior elements- — the ' frontals.' 



N.B. These constant bones are often the largest of the cranial elements in the 

 oviparous Vertebrata. 



The Ethmo-vomerine Sclerotome. 



1 a. The basal part or centrum — the ' ethmoid.' 



N.B. The mesial part of the ethmoid in Mammals, including the ' crista galli,' is 

 the central element of their most anterior sclerotome. In Birds this central 

 piece is very variably developed, and soon coalesces with the pre-sphenoid in 

 the Struthionidae. In Crocodiles, Chelonians, Lacertians, and indeed in most 

 Reptiles (except in some extinct forms, e. g. Dicynodon) (Huxley, Quart. Journ. 

 Geol. Soc. vol. XV. pi. 22 p. G55), it is cartilaginous or membranous ; and the 

 same might be said of the Ophidians and Batrachians, if Professor Goodsir's 

 views were correct'. In osseous Fishes it is usually ossified, — from one centre 

 in most species, but from two in the giant Sudis, and in the little Smelt 

 {Salmo eperlanus). 



1 b. Cortical element or elements — the single or double ' vomer.' 



2. The internal-lateral elements — the ' pre-frontals.' 



N.B. These form the lateral masses of the 'ethmoid' in Mammals, are often 

 cartilaginous in Birds, and are distinct and large in the Crocodiles and Batra- 

 chians ; but in the Chelonians, Lacertians, and Ophidians they coalesce, or are 

 connate with the ' lacrymals.' They are large and distinct in osseous Fishes. 



3. The external lateral elements — the ' lacrymals.' 



N.B. These bones are generally distinct in Mammals : there is scarcely an excep- 

 tion to this rule in Birds : they are large and distinct in Crocodiles ; in the rest 

 of the class of Reptiles they are not, as a rule, distinct from the pre-frontals. 

 In Fishes they form the sub-orbital chain of bones. 



4. The superior elements — the ' nasals.' 



N.B. These pieces generally meet each other at the mid-line, and are sometimes 

 single, in Mammalia ; in the Manatee they do not meet ; nor do they, as the 

 rule, in Birds — the Rhea and the Pigeons being exceptional. In Crocodiles 

 they are distinct and meet as in Mammals, but are seldom distinct from the 

 pre-fronto-lacrymal in Chelonia. In Lacertians they may be single or double ; 

 in Ophidians they are large and meet on the mid-line ; in Batrachians and in 

 osseous Fishes they are small and widely apart. 

 We have already spoken largely of the elements of the lower parts of the face : for 



' A careful investigation of the development of the ethmo-vomerine axis in Birds will, we have no doubt, 

 fully explain the meaning of this part of the skull iu the Ophidians and Batrachians ; to us it appears that it is 

 the pre-sphenoid which is abortively developed in all the lower Vertebrata. 



