ART. 4 CAMBRIAN CONCHOSTRACA— ULRlCH AND BASSLER 45 



BEYRICHONA TINEA PLANATA (Matthew) 



Plate 6, Figures, 3, 4; Plate 7, Figures 1-3 



Beyrichona planaia Matthew, Trans. New York Acad. Sci., vol. 14, 1S95, p. 134, 

 pl. 7, fig. 7. 



This is represented in the Matthew collection by four specimens, 

 three exactly alike, and one marked "B" proportionally longer on the 

 hinge line, this difference causing the posterior edge to range some- 

 thing like 80° to the hinge line. In the other specimens, which bear 

 the usual characters of the species or variety, this angle is very near 

 90°. Bearing this difference in mind, it appears that specimen B, 

 which is also the largest and best preserved of the four, is the original 

 of Matthew's Figure 7, on Plate 7, the specimen and figure agreeing 

 exactly in the posterior half. The anterior half, however, shows 

 differences, the figure being especially inaccurate in representing the 

 antero-ventral part of the outline as much more convex than we find 

 it in the specimen. His description says that the border is narrower 

 but we failed to notice any difference in this respect when compared 

 with B. tinea. 



Concerning the relations of B. planata to B. tinea, the types of the 

 two alone left no doubt as to their being very close. With the abun- 

 dant material in the Walcott collection, the case became complicated 

 by the finding of other slightly varying forms, all pointing, like B. 

 planata to B. tinea as the central type of a variable species. 



Variety C: Considering specimen B as the real type of the variety 

 planata, the other three specimens. A, C, and D, would represent a 

 second variety differing chiefly and probably solely in the more ver- 

 tical disposition of the posterior edge. Both differ from B. tinea s. s. 

 in the greater width of the cardinal portion of the slope from the 

 summit of the anterior ridge to the edge of the valve; in other words, 

 the anterior extremity of the carapace extends farther beyond the 

 anterior node than in B. tinea. The greater width of this slope lends 

 greater conspicuity to a low, generally undefined ridge that traverses 

 it vertically. The corresponding rib is more obscurely indicated on 

 the more abrupt anterior slope in B. tinea. As a rule the posterior 

 node or ridge is less definitely developed in these varieties than in 

 B. tinea but the difference is not constant. 



Variety D: Another variety is represented by numerous but not 

 very good examples in which the posterior margin is vertical as in the 

 second variety above, but the anterior slope is abrupt as in B. tinea, 

 while the postventral angle is sharply rounded as in the variety 

 triangula. From the latter, as also from B. tinea s. s., it differs 

 in its right-angled form, and in the comparative obsolescence of the 

 posterior and median nodes. 



