DEVONIAN FOSSILS 97 
(3) The median sulcus or carina is not so well developed as in 
C. alpha. This suleus is deep in C. alpha in both young shells (e.g., 
on specimen 27,220) and old shells, but at no time in the life history 
is the sulcus deep in C. beta. 
Comment. Study of the growth lines on these shells shows that 
the sub-orbicular outline of С. alpha and the sub-elliptical outline 
of C. beta are approximately the same throughout life, i.e., their 
outlines do not change in proportions during growth. The two 
species are readily distinguished by their outlines. 
Genus CONCHIDIUM Hisinger, 1799 
Conchidium polymitum sp. nov. 
PLILIETBLRII, Bios, 012.18: 
Type Material. Holotype consisting of steinkern of ventral 
valve, specimen 27,181 from locality G 22. 
Etymology. The trivial name is derived from the Greek word 
polymitos (= with many threads). It refers to the multistriate 
ornamentation of this species. 
Description of Holotype. Ventral valve very convex. Umbo 
high but not overhanging that of the dorsal valve. Shell thick; 
ornamentation of very numerous striae. Part of the original ealeie 
material of the shell is preserved and shows the shell substance 
to be impunctate. 
Interior with strong spondylium supported by a median septum 
about 1 cm. long. Measured posteriorly the spondylium is 6-5 mm. 
wide and 8 mm. high on the outside measurement, and 5-5 mm. 
wide and 6-5 mm. high on the inside measurement. The septum is 
high; it is thick where it joins the spondylium on the floor of the 
shell, then gradually thins both dorsally and anteriorly. A fine 
furrow continues the line of the septum for some distance; this is 
flanked on one side by a fine ridge which may be the result of 
crushing. As with the other fossils of this fauna, the holotype 
has suffered shearing. Genital markings are very distinct in the 
umbonal area of the valve. 
Comment. In the collection from Sandy’s Creek there are two 
dorsal valves of Conchidium (specimens 27,205 and 27,228), but 
these vary from one another, and at present there is no way of 
telling whether one or neither of these is the dorsal valve of C. 
polymitum. However, in both specimens the septal plates are 
long (half the length of the valve or more) and slightly divergent. 
From the series of specimens present, it is clear that C. poly- 
mitum was biconvex, subtriangular, and rectimarginate; also the 
G 
