BRACHIOPODA. 123 



sidenible amount of calcareous matter in it; no signs of punctation to be seen 

 with a half-inch objective. Valves convex, the lower valve varying in amount 

 of convexity with its habitat, but always more or less inllated. A small, sharp, 

 longitudinal septum rises from the center of the lower valve, of a subtriangular 

 shape, covering and hiding a small tubular perforation of the apex of the shell. 

 This perforation is very oblique, and from its internal opening a groove extends 

 backward nearly half-waj^ to the posterior l^order of tlie sliell inside. The an- 

 terior muscular scars meet in front of the septum and Ibrm a semilunar eleva- 

 tion with the points directed backward. The posterior scars in the lower valve 

 are small and widely separated. On the external surface the foramen appears 

 nearly in the middle of the shell and the furrow is continued anteriorly for a 

 short distance. (There is no furrow in my specimens outside behind the fora- 

 men, which is the only point of difference from Sowerby's figure.) Upper 

 valve convex, apex subcentral ; a slight median longitudinal callus internally. 

 There is no strongly impressed disc about the foramen as in Discinisca, though 

 slight traces of a differentiated area exist there."* 



Discina striata, Schumacher, is the only known representative of this genus. 

 (See Plate IV k, figs. 23, 24.) Its minute, almost imperceptible foramen (so 

 small that both Schumacher and Gould, not seeing it, referred the species to 

 Crania), and the arrangement of its internal impressions, separate it by a full 

 generic difference from its nearest allies. With Discina, therefore, we have at 

 present nothing to do among the palaeozoic brachiopods. 



The foregoing limitation of the genus Discina by Mr. Dall left all the other 

 recent species, currently referred to that group, without a designation, and for 

 these the author proposed the term Discinisca, witli the following definition :f 



"Sub-genus Discinisca, Dall,= Discina, Auct. Lower valve more or less 

 flattened, concave or compressed, Upper valve more convex ; apices of both 

 subcentral or subposterior. Lower valve with a snudl septum as in Discina, 

 behind which is an impressed disc or area, externally concave and internally 

 elevated. This is perforated by a longitudinal fissure, extending from a short 

 distance behind the septum nearly to the posterior margin, which is often 

 slightly indented behind it. Shell more or less horny in texture, minutely 

 tubulose. Type, Discina lamellosa, Broderip."J 



* Bull. Mus. Comp. Zool., vol. viii, No. 1, p. 3S). 

 t Bull. Mus. Oomp. Zool., vol. iii, No. 1, p. 37. 

 I Proc. Zool.. See, 1833, p. 124. 



