ANIMAL TO ITS SHELL IX SOME FOSSIL CEPHALOPODA. 93 



curving rather abruptly inward but still passing backward, it disappears at about 5'5 mm. 

 from the median line, exlni)iting, however, a tendency to again turn forward. The 

 portion of the scar which is visible has a pyrifortn shape ; its axis is at an angle of 

 about 45° with the median line of the dorsal area of the whorl, the '"stalk" being- 

 directed outward and backward ; its length is about 8'5 mm., and its gi'catest width 

 about 5 mm. 



The other scar is truncated by a fracture crossing it obliquely, so that only the 

 posterior portion of the impression is seen. It is somewhat darker than the surrounding 

 surface, and bounded anteriorly and laterally by a very slightly-raised line. Commencing 

 from the fracture at about 9 mm. in advance of the most anterior part of the last septum, 

 and at the same distance from the median line of the dorsal area, the anterior boundary 

 passes backward and very slightly outward ; then, curving round, comes to within 2 mm. 

 of the suture-line ; then, curving inward and forward, it is again intercepted by the 

 fracture at about 4 mm. from the median line. The anterior and outer boundaries of 

 the impression are sharply marked by a feebly-incised line, but the posterior boundary is 

 not quite so sharply defined. 



There is a very shallow and rather broad depression on the peripheral area about 5 mm. 

 in front of the summit of the siphonal (peripheral) saddle ; on either side of the median 

 line of the periphery, and at a distance of 3 mm. apart, a very faint line curves forwai'd 

 and towards the median line of the periphery, nearly meeting its fellow at about 3 mm. 

 anterior to the depression just mentioned. This possibly represents the anterior border 

 of the peripheral portion of the annulus. 



JiIgoceratid.'E, Neumayr, emend. Zittel. 



Arietites, Waagen, emend. Zittel. 

 Arietites ohtusus, J. Sowerby, sp.* — The muscular impression is well shown in an 

 example of this species from the Lower Lias of Lyme Regis, that was kindly lent to me 

 by Mr. G. F. Harris, F.G.S. The specimen is a fairly-complete internal cast, almost 

 entirely denuded of the test. Its dimensions are : — diameter of shell 101 mm. ; width 

 of umbilicus 43 mm. ; height of outer whorl 32 mm. ; ditto above preceding whorl 

 29 mm. ; greatest thickness (excluding ribs) 30 mm. ; ditto (including ribs) 33 mm. 

 The dimensions of the base of the body-chamber are : — height of whorl 26 mm. ; thick- 

 ness (excluding ribs) 22 mm. ; ditto (including ribs) 25 mm. The body-chamber is 

 complete and occupies the last half-whorl. The muscular impression is situated at the 

 base of the body-chamber and occupies the inner area of the whorl (PI. 19. figs. 8, 9, 10). 

 Its anterior boundary, represented by a fine incised line, is convex, its anterior portion 

 being 16 mm. in front of the anterior portion of the second lateral saddle of the last 

 septum. Passing backward its outer boundary is nearly parallel to, and at a distance 

 of 6 mm. from, the suture (of the shell). It can be traced backward for a distance of 



* Min. Con. vol. ii. p. 151, pi. clxviii. (l'6\'l)= Asteroceras ohtusum (J. Sowerby): fide Hyatt, 'Genesis Arietidse,' 

 1889, p. 201. 



