153 



The Ostracoda found in those deposits afford a means of discriminating Peculiarities 

 the layers, second only to the Brachiopoda. They are not nearly so num- the OsSwoda 



erous as the latter, or they would be even more valuable in this resoect th , e ? asal 



i *j 11 T i !-., ' Cambrian, 



as they show considerable liability to variation. The torms of Bradorana 



are specially abundant in the Lower Etcheminian strata, while Bradoria 

 and Schmidtella (?) are more common in the Upper. From their small 

 size these Ostracoda are easily overlooked, but their thick and strong 

 shells have resisted destructive agencies, and give examples that have 

 not suffered so much from distortion and pressure as some of the Bra- 

 chiopods. They possess some features of form and structure which are 

 peculiar. One notable feature is the position of the main muscle scar 

 Mr. E. O. Ulrich, who has given much attention to the study of the 

 Ordovician and Silurian Ostracoda seems to assume that the place for the 

 muscle is near the centre of the valve. At least he speaks of this as the 

 position of this mark in Leperditia* ; it is from the hinge to this point 

 that the sulcus or transverse groove extends in this genus. And if there 

 is any meaning in this connection in the central depression of the valve 

 a similar position for the muscle scar may be inferred for Primitia, Primi- 

 tiella, Isochilina, Kirkbya, Entomis and other genera. 



But in the Etcheminian species of Ostracoda and in many of those of 

 the Protolenus Fauna of the St. John Group, we have not been able to 

 find any in which the muscle scar is so placed. On the contrary many 

 examples occur in which the scar holds an anterior position near the hinge 

 line. This peculiarity would have given greater mobility to the valves, and 

 it is a fact that while in many cases we find the valves spread somewhat 

 apart, there are others in which they are spread out flat and yet retain 

 their normal position of contact at the hinge line. 



If there is any meaning in these furrows that extend from the hinge 

 as indicating muscular attachment there is a suggestion of a posterior 

 muscle, towards the posterior end of the cardinal line in the depression 

 that exists there in Beyrichona and Hipponicharion, and is faintly shown 

 in Escasona. But of such posterior muscle we have no sure evidence. 

 Of the anterior adductor muscle, however, there are plain indications on 

 the interior of many valves. 



It is clear that Ostracods having such a radical difference of structure Peculiarities 

 from those of a later time, must have had different habits of life, of h a bitet - 

 and among other peculiarities noted is that they usually occur solitary. 



*Lower Silurian Ostracoda of Minnesota, p. 633. 



