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



§ 2. The Clavicular Arch. 



(i.) Since the clavicular arch was figured in PI. Hawkinsi (' Geol. 

 Soc. Quart. Journ.,' 1874, p. 444), v. Zittel has figured the clavicular 

 hones in PI. laticeps (' Handhuch der Palaontologie,' vol. 3, p. 489) ; 

 hut, while the clavicles are clearly shown, the interclavicle is named 

 episternum. The most important evidence of this structure in Plesio- 

 sauridee, however, is to he seen in Plesiosaurus arcuatus (' Brit. Assoc. 

 Hep.,' 1839, p. 76 ; and ' Oat. Foss. Kept, and Amph.,' Part II, p. 

 163), preserved in the British Museum. From that specimen, No. 

 2028*, the character has heen attributed to Thaumatosaurus (loc. cit., 

 p. 159) : " Omosternum consisting of a large single plate, much ex- 

 panded transversely, with a wide and shallow anterior notch. "f The 

 anterior margin of the interclavicle in this specimen resembles in 

 contour that attributed to Eretmosaurus (' Geol. Soc. Quart. Journ.,' 

 1874, p. 445) in its wide open curvature ; but there is no evidence to 

 show whether the shoulder girdle, pelvis, and limbs in Plesiosaurus 

 arcuatus were constructed on the same plan as in PI. rugosus. There 

 is no doubt that the bone consists of three distinct elements united by 

 sutures. These are a median interclavicle and two lateral bones 

 which I regard as clavicles. On the visceral aspect the triangular 

 clavicles are separated from each other by the wide short posterior 

 median bar of the interclavicle, but the clavicles extend forward so 

 that only a narrow transverse bar of the "J"" sna P e( i interclavicle is 

 exposed in front of them, extending across the entire width of the 

 bone. The interclavicle is 10J inches wide, concave on its anterior 

 margin, 1\ inch from front to back at the widened extremities of the 

 cross-bar, and t 8 q inch in the same measurement towards the oblong 

 middle portion of the bone. The right anterior transverse limb of 

 the cross-bar is 4 inches wide ; the left limb is 3 inches wide. The 

 middle portion of the bone is 3£ inches wide and 2J inches in antero- 

 posterior measurement. The sutural line which defines the inter- 

 clavicle is sagittal, and consequently irregular. On each side of this 

 T-shaped interclavicle (fig. 2), in contact with the posterior margin of 

 its transverse bar aud the lateral margin of its short wide median stem, 

 is a large triangular clavicle which is directed backward and outward. 

 In harmony with the dimensions of the transverse bar, the right 

 clavicle is the wider. Anteriorly it is 4J inches wide ; it is nearly 

 6 inches long. The external border, which is slightly convex, is con- 

 tinuous with the truncated lateral termination of the interclavicle in 

 front of it. These external margins diverge outward as they extend 

 backward, so that the transverse measurement over the posterior 

 extremities of the clavicles is 14| inches. The postero-internal 



f Compare Sollas, ' G-eol. Soc. Quart. Journ.,' vol. 37, 1881, p. 457. 



