134 



Prof. H. G. Seeley. The Shoulder Girdle 



median line it can only have been by squamous approximation. Thus 

 arranged they would be inclined to each other. As preserved, each 

 clavicle is about 4 inches wide ; and on its inner border measures 

 2f inches from front to back, and at the external angle the corre- 

 sponding measurement is § of an inch. The anterior border is 

 straight ; the inner border is sinuous and unsymmetrical on the 

 opposite sides ; the posterior border is 3^- inches long and concave, 

 with the concavity broken on the inner third by a sharp prominence 

 which separates a slight inner concavity from the longer external 

 concavity. 



The external extremities of the bones are truncated and striated. 



The only specimen which distantly approximates to this in the 

 large size of the radius as compared with the small ulua is an 

 Elasmosaurian indicated in the Leeds Collection by the number 31. 

 In that also there is no trace of an interclavicle, but the shoulder 

 girdle is not perfectly preserved, and its clavicles are of dissimilar 

 form. If the scapulae in mature individuals of this species united in 

 the median line and extended back to the coracoids, then the fossil 

 would be Elasmosaurian, and possibly a species of Cryptoclidus. 



III. The Clavicular Arch in the ElasmosauridjE. 



§ 1. The Nature and Limits of the Family. 



When the Elasmosauridae was denned in 1874 its clavicular arch 

 was unknown and supposed to be wanting, and the family was 

 based upon the circumstance that the bones named scapulae met 

 each other in the median line, and were prolonged backward to unite 

 with the median processes of the coracoids in Elasmosaurus and 

 Golymbosaurus. I owe a knowledge of the clavicular arch in this 

 family to A. "N. Leeds, Esq., of Eyebary, who for twenty years has 

 collected the fossil Vertebrata from the Oxford Clay near Peter- 

 borough. In this family the cervical vertebrae have the ribs attached 

 by undivided articular heads. The carpal and tarsal bones are poly- 

 gonal and well ossified. The genera on which the family is based 

 are Elasmosaurus, Colymbosaurus, and Murcenosaurus (' Geol. Soc. 

 Quart. Journ.,' 1874, p. 436), none of which appear in the ' British 

 Museum Catalogue of Fossil Reptiles,' Part II. The only genera in 

 that enumeration which could be so referred are Polyptychodon and 

 Cimoliosaurus. Excepting Polyptychodon only, all English as well as 

 all American Elasmosaurian s have been referred to the latter genus 

 in the Catalogue referred to. Hence, as it will be presently shown 

 that the Elasmosauridae develop remarkable modifications of the 

 clavicular arch, which may be regarded as of generic importance, it 

 is convenient to determine as far as possible the synonymy of the 

 genera comprised in the family. 



