FROM THE LOWER LIAS OP CHARMOUTH. 



457 



notches, point to its connate nature ; its position in front of the 

 coracoids, between the prescapular processes of the scapula), points 

 to its clavicular origin : I regard it therefore as representing a pair 

 of fused clavicles, which repeating the behaviour of the coracoids, 

 have expanded into extensive plates over the ventral surface. An 

 interclavicular element appears to be absent ; there is no room for it, 

 except in the position conjecturally assigned to it by Professor 

 Seeley, who has suggested that it forms the anterior middle part of 

 the bone. This, however, is a position which it occupies in no other 

 known reptile, as it is always more or less posterior instead of an- 

 terior to the clavicles. Since writing the first part of this paragraph 

 I have been able to devote a few minutes to an examination (which I 

 wish could have been less hasty) of a loose specimen of Plesiosaurian 

 furculum, preserved in the British Museum, the same bone, I fancy, 

 that is figured as a sternum in Hawkins's monograph. It certainly 

 shows traces of sutures, and is marked on the surface by stria?, which 

 appear to indicate a median and lateral elements. It has a suggestive 

 resemblance to the clavicles and interclavicle of a Chelonian like, say, 

 Trionyx. But it differs considerably in form and appearance from 

 the furculum of our species ; so that it is doubtful how far it can be 

 used as a guide. Very possibly the furcula of different Plesiosaurs 

 may differ in composition, as they do in Birds, an interclavicle being 

 sometimes present and sometimes absent. 



There is another difficulty attending the interpretation of the fur- 

 culum ; and that lies in its position beneath the prescapular ends of 

 the scapulas, which overlap its posterior lapels. In all recent rep- 

 tiles the clavicles are superficial to the scapulas, while here just the 

 reverse is the case. This is proved by more than one well-preserved 

 specimen in the British Museum, showing the scapular processes 

 abutting on the body of the furculum, and also by Lord Enniskillen's 

 specimen of P. macrocephalus, which affords us a dorsal view of the 

 left clavicle overlapping the dorsal surface of the scapula (fig. 3). 



Fig. 3. — Diagram slioiving the left side of the Pectoral Arch of 

 P. macro cephalus, seen from behind. (The wing of the furculum 

 conceals the termination of the scapula.) 



Cl 



Scv -/-- 



