32 PKOCEE DINGS OF THE NATIONAL MUSEUM vol. 78 



anteriorly. Except W. ventrosa, in which it is situated relatively 

 farther forward than in the others, the variability in position with 

 respect to the middle of the valves would seem to be largely ac- 

 counted for by variation in the proportionate lengths of the anterior 

 and posterior halves. Regarding the eminence as an approximately 

 fixed point, and taking into account the matter of size of valves, the 

 proportional distance from its apex to the anterior edge is much 

 less variable than is the distance to the posterior edge. Hence, 

 when the valve is elongate, as in W. oblonga, W. longula, W. limatula, 

 and W. scitula, the apex is more or less anterior of the middle. On 

 the contrary, when the valve is relatively short, the apex is near the 

 middle or behind that point. 



A rather disconcerting fact is notable in W. leperditoides, W. nitida 

 and W. obsoleta. In these species, namely, the median eminence is 

 barely distinguishable or quite obsolete. After close study, however, 

 it has seemed indisputable that the first two are still close allies of 

 W. pulchella and W. apicalis, while the last can not be far removed 

 from W. oblonga. 



Respecting the morphological significance of the surface nodes of 

 Walcottella, the small elevation frequently observable in the antero- 

 cardinal angle is thought to be the homologue of the similarly placed 

 node in Bradoria, and of the "eye tubercle" in Leperditia and Iso- 

 chilina. The larger median eminence doubtless corresponds to the 

 "muscle spot" of the Ordovician and Silurian Leperditiidae and not 

 to the more ventrally situated but otherwise similar spine found 

 in the group of Isochilina typified by /. armata. The relations sug- 

 gested are further strengthened by the striking similarity in general 

 aspect presented by W. leperditoides and W. nitida to average examples 

 of Leperditia. Were it not for the obvious differences in the sub- 

 stance and other features of their respective shells, it would be dif- 

 ficult to point out convincing reasons for classifying these two Cam- 

 brian species under Walcottella and not Leperditia. 



Compared with other Cambrian genera of bivalved Crustacea, it 

 is at once distinguished from Bradoria by the median eminence. 

 Comparing certain species of each genus, essential agreement in 

 general outline is noted, but the more typical species of Bradoria 

 exhibit obtuse angulations in the anterior and posterior parts of 

 their outlines — sometimes also in the ventral part — that do not occur 

 in Walcottella. Matthew's genus Indiana, as herein defined, is prob- 

 ably also related, but the total absence of surface nodes in that 

 group of species is a sufficiently obvious distinction to render confu- 

 sion between them unlikely. It might occur only in comparing forms 

 like Walcottella leperditoides and the associated Indiana faba. An- 

 other useful, though perhaps not very important, difference is ob- 

 served in studying the outer surface of the shell. In Walcottella, 



