300 THE CARBONIFEROUS SYSTEM. 



This is probably the species referred by Mr Davidson, with doubt, 

 to R. pugnus (?) in his paper above referred to. 



Centronella Anna, Hartt (Fig. 99). — Shell orbicular, lenticular, 

 equilateral, inequivalve, the dorsal (ventral, Hall) valve being con- 

 siderably more arched than the ventral (dorsal, Hall). Dorso-ventral 

 diameter about half that of the width of shell — length about a quarter 

 of an inch. 



Fig. 99. — Centronella Anna, Hartt. 



[a) Shell natural size. (J) Internal loop. 



Ventral valve with lamellse which take their origin near together. 

 These lamellte separate slightly from one another until they are inclined 

 to one another at an angle from 30°-45°, when they curve towards the 

 mesial line, and meeting at a very acute angle, are prolonged back- 

 wards in a pointed arch to three-quarters — four-fifths the length of the 

 shell, the width of the arch being approximately one-half its length. 

 The planes of the lamellae are at first parallel, but their dorsal edges 

 soon become moderately inclined outward. The lateral bands are not 

 only bent toward the mesial line, but they are strongly curved, with 

 the convexity towards the ventral valve, the curve being slightly 

 greater than that of the valve. This loop supports on the dorsal side 

 a thin plate, whose plane coincides with the dorso-ventral and antero- 

 posterior diameter of shell, and a thin plate extends from the apex of 

 the arch forward (backward auct.) for about two-thirds its length. 

 This plate seems to be of uniform thickness throughout. At the point 

 of the arch the supporting lamella? are exceedingly slender. Tracing 

 them anteriorly, they are seen running along the ventral border of 

 the mesial plate, on each side, like a raised line. Increasing in width, 

 they separate themselves more and more along the dorsal margin 

 from the mesial plate, to whose ventral border they are attached for 

 its whole length. The plate has an outline similar to that of a 

 transverse section of a biconvex lens whose diameter is twice its thick- 

 ness, but in both the loops under examination there is on the dorsal 

 edge a notch which appears to be organic, and to correspond to that 

 of the loop of Centronella Julia, Billings. 



The mode of attachment of the mesial plate with the lateral bands 

 is very well shown in my specimens. Professor Hall has called atten- 

 tion to the strong resemblance between the loops of Centronella and 

 Rensselaria, but there is a much greater resemblance between the 



