ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 65 



(7 to 12) large and smaller rounded elevations, the largest and most 

 prominent of which occurs usually a little in front of the middle of the 

 ventral edge; with or without a distinct marginal flange. 



Genotype. — Polyphyma lapworthi Groom. Cambrian of England. 



Two of the four species referred to this genus have hitherto been 

 placed in the ostracod genus Beyrichia. The sole reason for the older 

 arrangement is that the valves have nodular elevations suggesting 

 the lobes of a Beyrichia. That the several nodes can not be correlated 

 with the lobes or nodes of a true Beyrichia, and that all other features, 

 notably the composition of the test, are very different, was not ob- 

 served except by Holm, who with his usual acumen denied the 

 ostracod nature of the Swedish Cambrian bivalved Crustacea and 

 suggested placing them with the Phyllocarida.^ For reasons given 

 in the introduction, the present authors regard the balance of the 

 evidence as favoring the Branchiopoda rather than either the Phyl- 

 ocarida or the Ostracoda. 



Compared with other genera, Polyphyma seems more nearly alhed 

 to Aluta, Beyrichona, and Hipponicharion than to any of the other 

 generic groups. Lepiditta and Isoxys also are suggested by their 

 shapes, but it is not believed that the relations to either of these 

 genera is at all close. At any rate the shell of Lepiditta is thought 

 to have been more calcareous, while the absence of surface nodes 

 affords an obvious criterion in separating the two groups. As to 

 Isoxys, its shell was probably not materially different, but consider- 

 ing its nontuberculated valves and the fact that its species grew to 

 such greater size, all thought of genetic relation to Polyphyma is 

 dismissed from the investigator's mind. 



Reverting to the true alhes of the genus, it is distinguished from 

 Aluta (as herein defined) and Beyrichona by its more nearly semi- 

 circular outline and numerous, comparatively variable tubercles. 

 Hipponicharion, possibly, is the nearest of these true alhes, but is 

 readily distinguished by the segregation of most of the nodes into a 

 continuous or medially divided submarginal ridge, which gives it a 

 very characteristic, regular aspect that is strikingly different from 

 the apparently irregular arrangement of the tubercles in Polyphyma. 



The adopted orientation of the valves is based on the correlation 

 of the largest of the ventral nodes with the antero-ventral node in 

 Aluta. This is located a httle to one side of the middle — presum- 

 ably the anterior — in Polyphyma. 



' Ilolm, G., Sveriges Kambr.-Silur. Hyolithidae, 1893, p. 110, foot note (Sveriges Geol. Unders., ser. C, 

 No. 112). 



2607—31 5 



