A^'IMAL TO ITS SHELL IN SOME FOSSIL CEPHALOPODA. 91 



Lytoceratid^, Neumayi", emend. Zittel. 



Lytoceras, Suess. 



Lijtoceras cornucopia;. Young & Bird, sp. — The impression of one of the slicll-muscles 

 has been observed in an example of this species in the British Museum (No. 37851) from 

 the Upper Lias of Whitby, Yorkshire (PI. 19. figs. 15, 16, 17). The dimensions of the 

 specimen are : — diameter 22 mm. ; width of umbilicus 8 mm. ; height of oviter whorl 

 8 mm. ; greatest thickness of the outer whorl 8 5 mm. Rather more than one-half of 

 the last whorl is occupied hj the body-chamber, the base of which is nearly circular, its 

 dorso-ventral and transverse diameters being each G mm. The amount of indentation by 

 the preceding whorl is very small. At the base of the body-chamber, and on the inner 

 area of the whorl, tliere is an oval impression (fig. IG), truncated on the inner side by the 

 ed-TC of the " impressed zone." The diameters of the portion that can be seen are 

 1-75 mm. and 2'0 mm. respectively, the longer diameter being placed transversely on 

 the whorl. The anterior and the posterior boundaries are both visible, the latter l)eing 

 almost close to the first auxiliary saddle ; but no trace of the annulus can be seen. 



On the peripheral area there is an elongated linguiform impression (fig. 17), which is 

 rounded and submucronate anteriorly, but is open posteriorly, each limb being situated 

 in each half of the siphonal (periphei'al) lobe. Its anterior portion is a little in advance 

 of the most anterior part of the siphonal saddle ; its posterior portion is slightly 

 contracted. Its length is 3'25 mm. and its width (between the limbs) '75 mm. 



In the British Museum Collection there is also a portion of the natural internal cast of 

 the body-chamber of another example of this species (PL 19. figs. 13, 14), from the Upper 

 Lias of Whitby, which exhibits the impression of the boundary of the right muscular 

 scar surrounded anteriorly and laterally by a dark-coloured, longitudinally-elongated, 

 oval area, a portion of which is slightly rougher than tlie rest. Its form is shown in 

 fig. 13. Only the right side of the base of the body-chamber is preserved, the heiglit of 

 the whorl here being 35 mm. At about 4'5 mm. anterior to the last septum, the incised 

 line indicating the position of the boundary of the muscular scar passes fi-om the suture 

 of the shell — /. e. the edge of the impressed zone — in a radial direction for about 7 mm. ; 

 then, turning abruptly backward nearly at right angles to its former course, it is 

 continued as far as the last septum. At the suture of the shell the boundary of the 

 oval dark-coloured area is 3'5 mm. anterior to the incised line just mentioned ; after 

 proceeding forward in an anteriorly-concave curve for about 2 mm., it passes in an 

 anteriorly-convex curve forward and outward for about 7 mm. ; then, after curving 

 gently backwai'd for about 5 mm., it can be traced nearly as far as the last septum, 

 having a direction nearly parallel to the edge of the impressed zone. 



Lytoceras fimbriahmi, J. Sowerby, sp. — The muscular impression is also shown in a 

 fragmentary example of this species in the British Museum (No. 20837) from the Middle 

 Lias of Kilsby Tunnel, Northamptonshire. The specimen consists of nearly half a whorl, 

 and is a natural cast of part of the body-chamber of a shell having approximately the 



