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MR. E. PHELPS ALLIS, J UN., ON 



subnasal, instead of prenasal in position. The palatine pro- 

 cesses of the palatoquadrate must, however, also take some 

 important part in the formation of this beak, for the anterior 

 dental plates, which it supports, lying as they do posterior to the 

 primary upper lips (Allis, 1917 b), must belong to the primary, 

 or palatoquadrate dental arcade. These plates cannot accord- 

 ingly be intermaxillary (premaxillary ) teeth, as Schauinsland 

 (1903, p. 14) suggests as possible in Callorhynchus, and they must 

 be either vomerine or vomero-palatine teeth according as the 

 term palatine is used to apply to teeth developed in relation to 

 the palatine process of the palatoquadrate or in relation to some 

 part of the palatoquadrate that lies posterior to that process. 

 The posterior dental plate of either side would then be either 

 a pterygoid or a palato-pterygoid plate, a pterygoid elem'ent 

 quite certainly being included in it. Further facts in favour of 

 considering the beak of Chimcera to be formed in part by the 

 palatine process of either side are : — 



1. That the external surface of the prenasal beak gives 

 articulation to the anterior end of a cartilage that is quite 

 unquestionably an anterior upper labial, as will later be explained, 

 and this anterior end of this labial is frequently, in the Selachii, 

 in contact with the dorsal surface of the palatoquadrate but 

 never in such contact with the neurocranium, as is also later 

 explained. 



2. That the beak of ChimcEra is traversed, on either side, by a 

 small canal which, in the specimen used for illustration, begins 

 on its dorsal surface by a single foramen and opens on its ventral 

 surface by three small foramina which lie internal to the anterior 

 dental plate. This canal is traversed by a branch of the nervus 

 maxillaris trigemini which descends over the lateral edge of the 

 nasal capsule and is evidently destined to innervate the anterior 

 dental plate and the related tissues. The corresponding nerve in 

 Mustelus, Ckamydoselachus, and Rata, and hence probably in all 

 of the Plagiostomi, runs forward along the external surface of 

 the palatoquadrate and then over its ventro-lateral edge, thus 

 being separated from the trabecule by the full width of the 

 palatoquadrate, and while it might become enclosed in the lateral 

 edge of the palatoquadrate it seems impossible that it could 

 become so enclosed in the lateral edge of any cartilage of trabe- 

 cular origin. 



The mandibula presents no special features that seem to 

 require consideration, but it may be mentioned that there are 

 two surfaces for the articulation with the palatoquadrate, the 

 antero-lateral one being an articular facet and the postero-mesial 

 one an articular head. It is also to be noted that the line of the 

 gape of the jaws, when the mouth is closed, is approximately 

 parallel with the line of the trabecular, the plane of the epal and 

 ceratal elements of the mandibular arch thus retaining its 

 primitive perpendicular relations to the trabecular and hence 

 being directed postero- ventral ly instead of ventrally. 



