94 MK. G. C. CRICK ON THE MUSCULAE ATTACHMENT OF THE 



11 mm., beyond which point the surface of the cast is fractured. About 2 mm. behind 

 this line, and nearly concentric with its anterior portion, tliere is another very faint line, 

 evidently marking a former attachment of the shell-muscle. On the inner area of the 

 Avliorl there are three or four fine, faint, longitudinal grooves, extending over three-fourths 

 of the length of the body-chamber; one of these, 2 mm. from the suture (of the shell), 

 extends the whole length of the body-chamber. 



On the periphery, on either side of the median line, there is a feebly-impressed 

 sigmoidal line (fig. 10), tlie anterior portion of which is about 11 mm. in advance of the 

 anterior portion of the external (peripheral or siphonal) saddle. Each line arises about 

 2 mm. from the median line of the periphery ; it passes away from this line, and backward 

 to about 8 mm. from the most anterior portion of the external (peripheral or siphonal) 

 saddle, and then disappears. These lines may indicate the position of the attachment of 

 the animal to this part of its shell, but no connection can be seen between these 

 lines and the impressions on the inner area. A line drawn from the centre of the 

 shell to the most anterior part of these lines on the periphery crosses the inner 

 edge of the outer whorl at a point 7"i3 mm. posterior to the anterior boundary of the 

 impression which is seen on the inner area. 



Posterior to these lines on the peripheral area there is a horseshoe-shaped marking 

 with nearly parallel sides, each of which passes posteriorly into each half of the sijjhonal 

 lobe. It is rounded anteriorly, and crossed in its length by several lines parallel to the 

 anterior border. Its length (above the small median saddle) is 11 mm., and its width 

 5 mm. This may, or may not, be connected Avith the muscular attachment of the 

 animal, but similar impi'essions have been observed in other forms. (See jEgoeeras 

 capi'icorn/iij/, p. 95.) 



Arietites rarlcostatus, Zieten, sp.*- — In the British Museum Collection there is an 

 example of this species (No. 0.4882) showing the muscular impressions (PL 20. figs. 10, 11). 

 The locality of the specimen is not recorded, but, judging from the matrix, it came pro- 

 bably from the Lower Lias of Somersetshire. The specimen is a well-preserved natural 

 cast with the following dimensions : — diameter 51 mm. ; Avidth of umbilicus 33 mm. ; 

 height of outer Avhorl 10 mm. ; greatest thickness of outer whorl (excluding ribs) 

 14 mm.; ditto (including ribs) 16 mm. The body-chamber occupies the whole of the 

 last whorl, and its posterior part {ah) can be removed from the rest of the fossil; its 

 transverse section is transversely oval, its thickness (excluding the height of the ribs) 

 being 10 mm., and its height 7 mm.; it was only slightly indented by the broad periphery 

 of the preceding whorl. The greater part of the muscular impressions is on the concave 

 dorsal area, i. e. on the impressed zone. On either side of the median line of this area 

 (fig. 11) there is a subti'iangular impression, having its broadly-rounded apex directed 

 forward. These impressions are about 1 mm. apart, that on the right side of the shell 

 being the more nearly complete. Each is bounded by a faint double fine. Commencing 



* F. Zieteij, Vcrsteia. Wiirt. p. 18, \>\. siii. f. 4 = Caloceras rarkostatum {Ziitien), Jidi Hyatt, Gen. Arietidae, 

 p. 144. 



