130 Prof. H. G. Seeley. The Shoulder Girdle 



and as they extend outward approximate toward the external contour 

 of the bone without meeting it posteriorly in a point. 



The ventral aspect of the clavicular arch is different (fig. 3) owing to 

 variation in the positions of the sutures between the bones. The inter- 

 clavicle no longer shows the "f"-sliaped contour of the visceral surface, 

 but is a wide curved bar with an irregular sagittal termination on 

 its postero-lateral extremities. This is owing to the method of its 

 squamous interlocking with the clavicles, which overlap its visceral 

 surface more in front, and overlap its ventral surface more behind, 

 where their pointed extremities nearly meet each other in the median 

 line behind the interclavicle, and in the inch of space from which 

 they are absent there is a slight distortion of the bone, and some 

 evidence of a median posterior notch. The triangular forms of the 

 clavicles are more marked on this aspect of the bones than on the 

 other. 



The most remarkable character here shown is the squamous sutural 

 interlocking of the three bones by which their shares in forming 

 the clavicular arch is definitely established. It is also shown by 

 different directions of the lines of growth in the clavicles and inter- 

 clavicles. 



An isolated clavicular arch in the British Museum, R. 1322, 

 presents a similar character and form, and shows in its sutures similar 

 evidence of composite character. It has been assigned to the species 

 named Plesiosaurus megacephalus (Stutchbury) in the British Museum 

 Catalogue. It has a similar resemblance to the anterior contour of 

 the interclavicle in PJretmosaurus, but I am aware of no evidence by 

 which the species is identified from this bone, beyond a general 

 resemblance to some specimens in the Bristol Museum. 



The correspondence of structure in these clavicular arches with 

 that figured in Plesiosaurus Haw'kinsi and Plesiosaurus laticeps is a 

 coincidence of plan, though the difference may indicate a sub-genus, 

 and shows, I submit, that the original definition of the bones was not 

 a conjectural suggestion, as stated by Professor Sollas, but a recog- 

 nition of sutures which separate the interclavicle from the clavicles. 

 And it seems to me a sound induction that whenever the margins of 

 the clavicular arch are concave in front and behind, those concavities 

 border the interclavicle, and whenever there are wings produced out- 

 ward and backward, as in the specimen now figured, those wings are 

 formed by the clavicles in all Plesiosauridae. 



(ii.) Sir R. Owen, in 1841 ('Brit. Assoc. Hep.,' p. 64), remarks on 

 the shoulder girdle of Pliosaurus : — " The pectoral arch owes its chief 

 strength to a pair of immensely expanded coracoids, having a broad 

 and short entosternal bone on their anterior interspace, and support- 

 ing the clavicles or acromion productions of the scapulae."* Subse- 



# I have examined the specimens in the Museum of the University of Oxford 



