LIASSIC ICHTHYOSAURS. 81 



upper jaw, of which twenty-five, or thereabouts, are implanted in the maxillary. In the 

 lower jaw there are about sixty teeth in each dentary (Tab. XXVIII, fig. 2, 33). A few 

 detached teeth in a portion of a large Icli, tenuirostris from the Lias at Pyx Hill measure, 

 each, 1 inch 4 lines, the enamelled crown being about a third of that length ; the cement- 

 clad root is 4 lines in diameter, rather thicker in proportion than in the smaller-sized 

 specimens of the present species. 



The vertebral column (Tab. XXVIII, fig. 1) agrees in general length with the charac- 

 teristic shape of the head. In the best preserved specimens it is nearly four times the 

 length of the skull. 



I have counted 156 vertebrae in a well-preserved column of the large specimen from 

 Pyx Hill ; in this the pinnigerous part of the tail was two feet in length, and had been 

 bent down iu the burial and subsequent petrifaction of the Sea-dragon at almost a right 

 angle to the trunk; this deflected part included sixty centrums, which seemed to be 

 relatively somewhat shorter as well as narrower than those of the trunk. At the bent 

 part of the column the margins of the terminal articular facets were slightly deflected, 

 and markedly raised from the level of the sides of the centrum, indicative of the degree 

 and frequency of flexure at this part. The fore-and-aft diameter of a post-abdominal 

 vertebra in an average-sized Tenuirostral is 13 lines, the vertical diameter being 2 inches 

 6 lines. The terminal articular surfaces of the centrums are more unifomily concave 

 than in the previously described species. I have not found in Ich. tenuirostris more than 

 two hypapophyses at the fore part of the column, one wedged between the basioccipital 

 and the atlas, the other between the atlas and axis. This more simple apparatus for fixing 

 the immediate support of the skull suggests an accordance with the lighter and more 

 slender character of that part. The centrums gradually increase in fore-and-aft dimen- 

 sions to the pelvic region, and do not begin to decrease in size till about ten vertebrae 

 beyond the pai-t forming the base of the long caudal region. 



The ribs soon become long and slender as they recede from the head, and increase in 

 length to near the hind end of the abdomen ; thence they shorten less gradually than 

 usual. Forty-five pairs of the long and regularly-curved ribs show the external longi- 

 tudinal groave. 



The parial fins (Tab. XXVIII, figs. 4, 6) show a somewhat less disparity in the size of 

 the pectorals and ventrals than obtains in Ich. communis and Ich. intermedins. Their 

 framework has fewer and larger bones, and the fore paddle impresses one with its massive 

 proportions compared with the vertebrae. The clavicles are relatively more slender than 

 in Ich. communis, but of the usual form, diminishing at the two extremities. The scapula 

 is relatively larger than in Ich. intermedius, and is thicker and more expanded at the 

 humeral end ; its fore border is moderately concave and longer than the hind one. The 

 coracoid (ib., fig. 3, 52) has a broad neck supporting a large and thick scapulo-humeral 

 articulation ; it has a deep and narrow anterior notch, and a shallow posterior emargi- 

 nation. In a well-preserved specimen of the present species, in the Philosophical 

 11 



