BRACHIOPODA. 423 
Strophomena 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 lamellz are the variously defined adjustors. 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 
strie 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 striz, 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 O. (D.) pectinella 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 strize have become plications and have a tendency towards 
multiplication, while the general form and muscular scars are essentially those of 
O. (D.) deflecta. For further modifications of O. (D.) pectinella, see O. (D.) meedsi and 
its variety germana, and O. (D.) subquadrata. 
