ANIMAL TO IT.S SHELL IN SOME FOSSIL CEPHALOPODA. 95 



at the most anterior part of the antisiphonal (or antiperipheral) saddle and at a point 

 0'5 mm. from the median line, the boundary of the more nearly complete impression 

 2)asses forward and outward until it is 3 mm. in advance of the last septum and about 

 2 5 mm. distant from tlie median line ; then, turning backward and passing still outward, 

 it apj)roaches the last sejitum, and nearly (but not quite) meets the inner portion of the 

 lateral saddle, where it disappears, its place of disappearance being 4*5 mm. from the 

 median line of the dorsal area or of the impressed zone. The outer boundary of the 

 muscular impression is on the edge of the umbilicus, and therefore in a lateral view of 

 the Ammonite only the outer backwardly-directed portion of the impression can be seen. 

 There is a corresponding and similar impression on the other side of the specimen, but it 

 is not so complete, owing to a fracture of the fossil. There are no traces of the annulus. 



JiIgoceras, Waagen, emend. Zittel. 



JEcjoceras cajjricornum, Schlotheim, sp. — A portion of the im2")ression of the muscular 

 attachment of the animal has been observed in an example of this species from the 

 Lias of Cheltenham, that forms part of the British Museum Collection (No. 67929). 

 The specimen is a well-preserved natural cast, bearing portions of the test in a soft, white, 

 friable condition (PI. 20. fig. 2). By removing this with a stiff In'ush from near the 

 base of the body-chamlier, the muscular impression is well shown on either side. The 

 dimensions of the specimen are as follows : — Diameter of shell (including ribs) 47 mm. ; 

 ditto (excluding ribs) 44 mm. ; width of imibilicus 20 mm. ; height of outer whorl 

 (including ribs) 15 mm. ; ditto (excluding ribs) 14-5 mm. ; thickness of whorl (including 

 ribs) 17 mm. ; ditto (excluding ribs) 14-5 mm. At the base of the body-chamber the 

 height and thickness of the whorl are respectively 11 and 13 mm. including the ribs, or 

 10 and 10"5 mm. excluding the ribs. The outline of the muscular impression is seen on 

 either side at the base of the body-chamber and quite close to the suture of the shell. 

 It is convex anteriorly, its anterior boundary being 6 mm. in front of the most anterior 

 jwrtion of the second lateral saddle ; its outer boimdary is nearly parallel to, and at a 

 distance of 2 mm. from, the edge of the impressed zone, and passes jiosteriorly on to 

 the outermost branch of the second lateral saddle. Its anterior boundary exhibits a 

 tendency to curve forward at the suture of the shell. In the middle of the jieripheral 

 area there is a tongue-shaped scar, which is rather more than twice as long as wide, 

 has nearly parallel sides, and is rounded anteriorly ; each side of it just touches the 

 boundary of the siphonal lobe. The scar seems to have been covered with a thin film of 

 shelly matter, for j)art of this has been broken away from the anterior portion so as to 

 expose a black layer beneath. This scar would be situated in the immediate neighbour- 

 hood of the siphuncle, and may or may not have had anything to do with the muscular 

 attachment of the animal. Its length above the extremity of the small median saddle 

 is about 6 mm., and its width is 1"5 mm. A precisely similar scar has been observed in 

 Arietites obtiisns*. 



* See ante, p. 93. 

 SECOND SERIES. — ZOOLOGY, VOL. VII. 14 



