Page:Proceedings of the United States National Museum, Volume 76.djvu/135

Rh laterally. In these cases the original convexity of the valvelike portions of the carapace was reduced to almost paper thickness, the originally high keel being more or less displaced from the normal with attendant crowding together of the more dorsal and ventral portions of the thin shell. In many other instances the two valves were buried in more or less normal vertical position that resulted in the two valves being pressed flat dorso-ventrally so that the dorsal line traverses the middle of the fossil and the circumference or periphery represents the original convexity of the conjoined valves. In such cases, moreover, the normally converging ventral portions beyond the keel were folded back underneath the diverging dorsal halves of the valves. In such dorso-ventrally compressed specimens the periphery usually is made by the serrated keel.

Genotype.—Tuzoia retifera Walcott, 1912.

Stratigraphic range.—Lower and Middle Cambrian.

Geographic distribution.—Middle Cambrian in Manchuria and British Columbia. Lower Cambrian, British Columbia, and Pennsylvania.


 * Tuzoia retifera, 1912, Smithsonian Misc. Coll., vol. 57, no. 6, p. 187, pl. 33, fig. 2.

When Walcott first described this species in 1912 in the fourth preliminary paper based on the 1910 and 1911 collections from the Burgess shale quarry, he still had only the single specimen illustrated. Subsequently, among more than a score of additional specimens that appeared, some preserving the margins, certain ones apparently represent this species and consequently a more complete description may now be drawm. After careful scrutiny of the specimens that have marginal spines and of the original type I have come to the conclusion that in the latter the marginal spines have been broken away and that it was not originally without them.

The original description of the species relies exclusively on the illustrations to present the characters of the animal. A formal description even now will add nothing to what can be ascertained from the present illustrations, particularly when these are studied in conjunction with the foregoing generic outline. Several characters may be emphasized by being specifically pointed out here. First, the three posterior marginal spines, the central one in direct line with the keel, may be noted. One or two smaller spines occur forward of these. Along the margin, between both sets small spines or crenulations mark the edge. None of the specimens available permit a definite determination of the presence or absence of spines along the dorsal line where we usually find them in other species.