APHELECERAS TROCHLEA. 71 



Description. — " Discoid, of two and a half rapidly enlarging whorls, almost 

 entirely exposed in an umbjlicns with vertical sides ; sides of the shell nearly flat, 

 sloping with slight convexity from the rectangular edge of the umbilicus to the 

 narrow, deeply concave peripher}^ which is bounded by two acutely angular 

 edges ; mouth widely trigonal, notched at apex by the concavity of the periphery ; 

 septa moderately numerous, having a broad backward curve on the sides, and a 

 more abrupt one on the periphery, the two separated by an acute forward 

 angulation on the lateral edcre of each side ; last chamber occupying nearly half 

 of the body-whorl. Diameter five inches, proportional diameter of umbilicus i-q^, 

 antero-posterior diameter of mouth -^^^q, width of penultimate whorl Y(^, width 

 of mouth at outer edge of umbilicus -j^^, width of concave periphery at mouth 

 -i^o%"(M'Coy, 1855). 



Remarks. — The species here described by M'Coy, first in his ' Synopsis ' and 

 afterwards in the ' British Palaeozoic Fossils,' is represented in the " Griffith 

 Collection" (Science and Art Museum, Dublin) by the fragment of a body- 

 chamber figured in the first of these works {loc. cit.), the only specimen found, up 

 to the present time, in Ireland. The more perfect specimen, above described, 

 came from Kendal, Westmoreland. 



The fragment is a cast of part of the body-chamber with the anterior portion 

 broken off; it formed part of a very large individual, as the following measure- 

 ments indicate : height of whorl (dorso-ventral) at anterior end 62 mm., at 

 posterior end (a distance of 100 mm.) 50 mm., width of pei-iphery at anterior 

 end 24 mm., at posterior end 17 mm. Upon a chord of lOO mm., subtending the 

 convex side, the greatest curvature is 15 mm. 



The fragment is broken away on one side; on the other the side is very 

 slightly convex and slopes gently from the umbilical to the peripheral border. 

 The periphery is deeply concave, and, judging by the acute edge of the cast on 

 each side of it, had prominent keels. A fragment of the test remains upon the 

 periphery; it is almost cov^ered by an encrusting Polyzoan ; fine transverse lines 

 of growth can, however, be seen bending backwards, but there are no longitudinal 

 lines. 



This species bears a strong resemblance to A. exaratas, de Kon., sp.,^ a 

 circumstance which was not overlooked by de Kouinck in describing his species. 

 The points of difi'erence between them appear to be in the relative width of the 

 peripheral and dorsal areas; thus in A. exaratus these areas are more nearly 

 equal in width than in ^1. trochlea, or, in other words, the sides are more divergent 

 from the periphery to the umbilical edge in the latter species than they are in the 

 former. De Koninck also remarks that the form of the section in A. exaratus is 



^ " l'"auue Calc. Curb. Belgique," pt. 1, p. 120, pi. xxv, figs. Iff, h. 



