214 C. E. Beecher — Koninckina and related Genera. 



at an angle of about thirty degrees with each other, opening 

 toward the ventral side (tig. 6). On account of the shallow 

 and deeply concave region occupied by the entire animal, the 

 coils necessarily are directed ventrally, and the height of the 

 cone is very slight. As actually measured in a specimen (fig. 

 6), the diameter of the cone of spiral lamellae is 3'6 mm and its 

 height is about l mm . 



Figure 10 illustrates a small translucent specimen, showing 

 the form and disposition of the spiral ribbon at this period of 

 growth. No fact of special consequence is exhibited, except 

 that the development of the spiral apparently agrees with that 

 adduced for other genera. It will be noticed also, that the 

 comparative area occupied by the simple coil of one and one- 

 half volutions is as great as that in a mature specimen in which 

 the number of volutions has increased to four. 



The spiral impressions and vascular lines on the interior of 

 the body of the dorsal valve have been so often studied and 

 illustrated that there only remain for consideration the special 

 features within the beak and umbonal regions. On account of 

 the smallness of the specimens and the partial obscurity of some 

 of the markings, all the details which should be observed have 

 not satisfactorily been made out, and this is especially true of 

 the muscular scars, as no well-defined muscular areas can be 

 detected in the specimens examined. It is supposed, however, 

 that the poorly defined regions, indicated by a in fig. 7, at the 

 ends of the vascular trunks v, represent the adductor scars, but 

 the} 7 are so involved with the septum and vascular markings as 

 to be very indeterminable. The septum s begins at the beak, 

 and is angular over the umbonal region, and rounded over the 

 body of the valve, terminating in the anterior third of the 

 length of the shell. At x is indicated the articular indentation 

 for the loop, as previously mentioned. The two widely diver- 

 gent cardinal processes j become merged into the general sur- 

 face of the valve before reaching the hinge extremities. Under 

 their apices are situated the dental sockets b, and on the sum- 

 mits above are the bases of the processes supporting the rib- 

 bons. 



Some notice should be taken of the species described by 

 Swallow as Koninckina Americana* from the Kaskaskia 

 group, since it is the only American form which has been re- 

 ferred to the genus. Only the ventral valve was observed, 

 and was described as having a punctate shell structure, and a 

 "few short depressed spines near the borders," neither of 

 which characters are found in Koninckina, but are so evidently 

 productoid in their nature that the species probably should be 



* Transactions of the Academy of Science of St. Louis, vol. ii, p. 94, 1863. 



