BRACHIOPODA. 423 



Strophoniena Incurvata.] 



triangular in outline, nearly horizontal, compared with that of the other valve, and 

 marked by a number of parallel lines of growth; delthyrium widely triangular, 

 covered from less than one-half to more than two-thirds its length by a depressed- 

 convex deltidium, which is broadly excavated along the anterior margin. Interior 

 with well developed dental plates, which join the outer elevated margin of the short, 

 more or less quadrate-lobate muscular depression. In the center of this depression 

 are the small, elongate, centrally divided adductors; on the outside of these and 

 occupying the greater portion of the muscular area, are the diductors. At the base 

 of the dental lamellae are the variously defined adjusters. Immediately beneath the 

 cardinal area, and originating on each side of the muscular depression, are numerous 

 antero-laterally directed genital markings. Vascular sinuses indistinct. Interior 

 surface of the valves along the anterior margin with radiating, centrally channeled 

 striae occupying the depressions between those of the outer surface. 



Dorsal valve evenly convex medially, with a more or less reflexed or concave 

 portion on each side and immediately in front of the cardinal line. A defined narrow 

 depression has its origin at the apex, which is soon merged into the shallow, though 

 sometimes deep, mesial depression. Cardinal area striated, comparatively wide for 

 species of Orthis. Delthyrium broadly triangular and covered by a chilidium at the 

 apex, which continues along the walls of the delthyrium as separated plates. Crural 

 plates short, but prominent, originating at the inner ends of the walls of the delthy- 

 rium. The entire rostral cavity is occupied by a subhemispheric deposit of shell 

 matter, upon the posterior surface of which is situated an elongate, linear, crenulate 

 cardinal process, the anterior portion terminating in a short, broadly rounded median 

 septum. On each side of the latter are situated the depressions of the adductor 

 muscles, the posterior pair being most conspicuous. 



This species has been regarded, until quite recently, as a true strophomenoid 

 shell. The contour of the valves, elongate hinge-line and fine striae, combined with 

 more or less large deltidia, are doubtless strophomenoid features; yet the cardinal 

 and articulating processes and the muscular arrangements are decidedly orthoid 

 characters. Since these features are regarded as of greater importance than the 

 former, this species should be referred to the subgenus Dinorthis, In the line of devel- 

 opment between 0. (D.) deflecta and 0. (D.) pedinella some links are missing. In the 

 second species there is still a small deltidium in the ventral valve, which has been 

 reduced in the dorsal valve to linear ridges bounding the delthyrium. The cardinal 

 process is larger, the striae have become plications and have a tendency towards 

 multiplication, while the general form and muscular scars are essentially those of 

 0. (D.) deflecta. For further modifications of 0. (D.) pectinella, see 0. (D.) meedsi and 

 its variety germana, and 0. (D.) subquadrata. 



