part 3] FEOM THE WEALD CLAT OF BERWICE: (sTJSSEX). 297 



manifested very early, so that even in the Upper Liassic Micro- 

 cleidus Jiomalospondyhis there ah*eady appears an Elasmosaurian 

 shoulder-girdle of a type nearly as advanced as that found in 

 Oryptocleidus, Murcdnosaurus, etc. from the Oxford Clay : that 

 is to say, the enlarged ventral rami of the scapulae form a median 

 symphysis continuous posteriorly with the symphysis of the cora- 

 coids, while the reduced clavicular arch has already become 

 functionless or nearly so, and lies within and upon the anterior 

 scapular expansion. The retention of the primitive condition in 

 Leptocleidus (and probably in Brancasaurus, if, as seems likely, 

 Wegner's interpretation of its shoulder- girdle is erroneous) may be 

 a consequence of the freshwater, probably fluviatile, habitat of this 

 Plesiosaur, which resulted in its leading a life sheltered from the 

 great competition that seems to have resulted in more rapid 

 evolution among the marine Plesios,aurs. A somewhat similar case 

 occurs among the Cetacea, of which the freshwater Platanistidse 

 retain nuinerous primitive characters derived from their Squalodont 

 ancestors. This circumstance H. Winge^ explains as the con- 

 sequence of their comparatively sheltered habitat in rivers and 

 estuaries, where they evaded crowding-out by the higher Cetaceans- 

 It must be admitted, however, that the clavicular arch is not 

 reduced in all cases in the marine forms : for instance, in Dolicho- 

 rhyncJiops osborni Williston, from the Niobrara Chalk of Kansas, 

 the clavicles and interclavicle are of relatively large size, and 

 the general structure of the shoulder-girdle is not unlike that of 

 Leptocleidus although the form of the interclavicle is different. 

 Possibly this Cretaceous species may have originated from a fresh- 

 water form, which had secondarily adopted a marine life at a 

 comparatively late period. 



The specimens described in this paper, which is published b}^ the 

 permission of the Trustees of the British Museum, are all pre- 

 served in the Greological Department of that institution. 



EXPLANATION OF PLATES XIV & XV. 



Leptocleidus swperstes, gen. et sp. nov. 



Plate XIV. 



[All figures, except 5, are a third of the natural size.] 



Fig. 1. Posterior portion of skull, lateral view. 



2. The same, palatal view. 



3. Centrum of an anterior cervical vertebra. 



4. Centrum of a middle cervical vertebra. 



5. Posterior cervical, thoracic, and anterior dorsal vertebrae. A quarter 



of the natural size. 



[col. = columella ; d. = diapophyses ; -i.t^.fZ.^: intervertebral disc ; ma;.=maxilla ; 

 n.g^J.^neural spine; oc.c. = occipital condyle; or&. = orbit; pal.^pala- 

 tine ; par. = parietal ; j"^^- — P^-^^sphenoid ; p./. = P prefrontal ; pin.— 

 pineal foramen; p.i.f. == posterior interpterygoid vacuities; pi.=: ptery- 

 goid ; q. = quadrate ; r. — cervical rib ; sq. — squamosal.] 



^ ' Udsigt over Hvalernes indbyrdes Slsegtskab ' Vidensk. Medd. Dansk 

 Naturh. Foren. vol. Ixx (1919) p. 104. 



