440 ■ Canadian Record of Science. 



OSTRACODA. 



The Ostracoda found in those deposits afford a means 

 of discriminating the layers, second only to the Brachio- 

 poda. They are not nearly so numerous as the latter, or 

 they would be even more valuable in this respect, as they 

 show considerable liability to variation. 



From their small size they are easily overlooked, but 

 their thick and strong sliells have resisted destructive 

 agencies, and give examples that have not suffered so 

 much from distortion and pressure as some of the Brachi- 

 opods. 



They possess some features of form and structure which 

 are peculiar. One notable feature is the position of the 

 main muscle scar. 



Mr. E. 0. 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 Primitiella 

 Isochilina, Kerbya, 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 great 

 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 connection. 



* Lower Silurian Ostracoda of Minnesota, p. 633. 



