100 ME. G. C. CEICK OX THE MUSCULAE ATTACHMENT OE THE 



inner area of the whorl such as has been observed in Distichoceras Bcmgieri (see below) 

 and Hecticoeeras hecticmn (see p. 98). 



The other specimen (No. 896S a) has the following- dimensions, exclusive of the 

 periphei'al spines, which in this example are comparatively small : — diameter of the shell 

 11 mm. ; width of umbilicus 2 mm. ; height of outer whorl 5-5 mm. ; thickness of ditto 

 2'5 mm. At the base of the body-chamber the height of the whorl is 4"25 mm. 

 (exchiding spines), and the thickness 2'5 mm. The state of preservation of this- 

 specimen is quite similar to that of the one just described ; the dark line near the base 

 of the body-chamber, which is most prol)ably the anterior boundary of the mxiscular 

 attachment, is very like that in the previous specimen, but is slightly more waved on 

 the lateral area. In this example also there does not appear to be any forward 

 prolongation of the muscular impression on the inner {not impressed) area of the whorl. 



The lack of any forward prolongation in this species seems to be confirmed by a 

 specimen (No. 19536 e) in the British Museum Collection, from the Drift of Braunstou, 

 Northamptonshire, in which there is au indication of the anterior boundary of the 

 muscular attachment near the su.ture of the shell, and in this example also it is only a 

 short distance anterior to the last septum. 



Distichoceras, Munier-Chalmas. 



Bistichocoras Bangicri, d'Orbigny, sp. — In the British Museum Collection there are 

 two examples (No. 5140 a, b) of this species which clearly display the form and position 

 of the muscular attachment of the animal to its .shell. The species occurs in the 

 Oxfordian, but the locality of these specimens is not recorded. 



In the larger example (No. C. 5140 a) rather more than one-half of the last whorl is 

 occupied by the body-chamber (PI. 20. fig. 8). Its dimensions (not including the spines 

 on the edges of the periphery) are : — diameter 17 mm. ; width of umbilicus 4'5 mm. ; 

 height of outer whorl 8 mm. ; greatest thickness (which is at a very short distance from 

 the umbilicus) 4*5 mm. At the base of the body-chamber the height of the whorl is 

 5 mm., and its thickness 3"5 mm., the transverse section being sublanceolatc. Measured 

 along the curve of the inner portion of the lateral area, the anterior boundary of the 

 muscular attachment can be seen as much as G'5 mm. in advance of the last septum. 

 After passing backward at a distance of 1'75 mm. from, and parallel to, the inner edge of 

 the whorl for a distance of about 5 mm., the boundary then passes outwards in a shallow 

 curve towards the pcrijihery, just before reaching which it turns slightly forward. There 

 does not appear to be a definite line at the anterior extremity of the forward prolonga- 

 tion of the impression, but the area included between the boundary of the muscular 

 attachment and the inner edge of the whorl is a little roughened, the surface of the 

 specimen outside this line being smooth and polished. Where the boundary turns 

 outward there are several fine incised concentric lines. It is to be observed that the 

 boundary of the forward prolongation of the muscular attachment does not coincide Avith 

 the longitudinal groove running along the middle of the lateral area, but is situated 

 between the inner margin of the whorl and this groove. 



