THE PLESIOSATJEIA. 211 



intermedium, and fibulare, in the proximal row of tarsal bones; 

 1,2, 3, distal tarsal bones; 3It. metatarsus; Ph, phalanges— E, 

 the pectoral arch : Sc, scapula ; C\ coraf-oid ; n, clavicles and inter- 

 clavicle (?). — F, the pelvic arch : Pb, pubis ; //, ilium; Is, ischium. 



tlie rilj join, is produced forwards, so that tlie cervical ribs 

 of the Plesiosauria have a strong general resemblance to 

 those of the Crocodilia. In the posterior part of the neck 

 and the anterior part of the dorsal region, the ribs become 

 somewhat longer, and lose their anterior j)rocesses, gradttally 

 acquiring the rounded and curved form of ordinary ribs. 

 Their proximal ends remain simple, and the facets, with 

 which they articulate, become raised, and thrown outwards > 

 as transverse processes, developed from the arches of the 

 vertebraj. (Fig. 68, C.) 



In the anterior dorsal verfebrae, these transverse processes 

 rapidly acquire their full length ; and they are continued 

 under this form, descending somewhat lower upon the arches 

 of the vertebrae towards the sacrum, to the end of the dorsal 

 region. The neural spines acquire greater length, the 

 zygapophyses are well developed, and the articular sur- 

 faces of the centra retain the form which they possessed 

 in the cervical region. There are usually between twenty 

 and twenty-five dorsal vertebrae. The sacral vertebrae are 

 two, and resemble the others, except that the sacral 

 ribs are large and broad for the attachment of the ilium. 

 The caudal vertebrae, usually between thirty and forty 

 in number, become, as usual, reduced to little more than 

 centra at the end of the tail ; but, in the fore part of the 

 tail, they have well-developed spines and articular pro- 

 cesses, with ribs which become ankylosed to the bodies of 

 the vertebrae, only late in life. "Well-developed chevron 

 bones are attached between the ventral margins of succes- 

 sive centra of the caudal vertebrae. 



As has been mentioned, there appear to be no sternal 

 ribs, but there is a well-developed system of ossifications of 

 the wall of the abdomen, arranged in transverse rows from 

 before backwards; each row consists of a median bone, 

 slightly bent upon itseK, thick in the middle, and thin at 



