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Leptella, Hall. L892. 

 (Plate 18, FlgB. 10 13, 

 Shells small, normally concavo-convex. Outline semicircular 

 or semielliptical. Hinge-line straight, making the greatest 



diameter of the shell. Pedicle-valve evenly convex, cardinal 

 area moderately high, delthyrium for most of its extent covered 

 by a convex plate; teetii inconspicuous and unsupported by 

 Lamellae; muscular impressions undetermined. Brachial valve 

 slightly concave ; cardinal area comparatively high; the delthy- 

 rium tilled by the cardinal process, which, as viewed from 

 behind, is divided by a median groove for its entire height. On 

 the cardinal margin this process is double but less conspicuous 

 than the crural plates, which are arched and highly elevated 

 above the hinge-line. They are short, terminate abruptly, and 

 inclose deep sockets. The visceral area is flattened or concave, 

 its anterior margin forming a double curve, from which line the 

 surface of the valve is abruptly deflected. This visceral area is 

 divided by a broad median ridge and its surface covered with 

 line, sharp radiating lines which end at the line of deflection. 



Type, Leptella sordida, Billings. 



Distribution. Cambrian — Lower Silurian. 



Leptsena, Dalman. 1828. 

 (Plate 13, Figs. 1-4, 6-9.) 



Synonym ; L< ptagoniu, McCoy, 1844. 



Shells plano-convex when young, concavo-convex at maturity ; 

 convexity normal. Surface covered by conspicuous concentric 

 corrugations or wrinkles over the flatter portions of the valves. 

 Where these cease the surface is more or less abruptly and often 

 rectangularly deflected, forming a conspicuous anterior slope. 

 The whole exterior is covered with fine, even, radiating, thread- 

 like, tubular strias, which, in well-preserved specimens, are crenu- 

 lated by finer concentric stria\ Outline transversely subquadrate 

 or semioval. Hinge-line straight, its length making the greatest 

 diameter of the shell; extremities often subauriculate. Cardi- 

 nal area narrow, slightly wider on the pedicle-valve, not denti- 

 culate. Tn the pedicle- valve, the delthyrium is covered by a 

 convex deltidium, perforated at the apex by a foramen which is 

 closed at maturity or encroaches upon the apex of the valve. 

 1891. 75 145 



