LIASSIC FORMATIONS. 123 



seems to be rather longer, proportionately, than usual : in the subject of fig. 2 it is 

 2^ inches in length. 



In the composition of the lower jaw, I have noted that the angular element extends a 

 short way in advance of the surangular before disappearing externally. The point of 

 the surangular enters a notch in the hind part of the dentary, about half an inch 

 anterior to a line dropped from the fore part of the nostril. The angular disappears, as 

 usual, between the splenial and dentary, not between the splenial and surangular. The 

 splenials contribute a small proportion to the mandibular symphysis (PI. XXV, fig. 3, 32'). 



The teeth (PI. XXIV, fig. 8) conform in relative size and slenderness of crown with 

 the slenderness of the bones wielding them. Those at the fore half of the jaws incline 

 more backward than usual, and hardly assume a vertical position in the maxillary and 

 post-mandibular regions. I have counted from sixty-five to seventy on each side of the 

 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 (PI. XXXII, fig. 2, 33). A few 

 detached teeth in a portion of a large Ich. 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 (PL XXXII, 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 vertebras 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 in 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 uniformly 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 vertebras 

 beyond the part 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 

 17 



