3G8 anatinidjE. 



Sjiecies of Thetis. 



elliptica, Becluz. Korenii, Loven. 



granulata, Nyst and West. quadrata, Hinds. 



liyalina, Sow. 



Genus TYLERIA, H. and A. Adams. 



Shell oblong, equivalve, valves thin, nearly membrana- 

 ceous, covered with a thin epidermis, rounded anteriorly, 

 gaping and slightly produced posteriorly. Hinge composed 

 of a cartilage-pit in each valve ; cartilage internal ; liga- 

 ment partly external; a calcareous lamina extending from 

 the cartilage-pit anteriorly, as far as the front muscular scar, 

 supported in its length by calcareous septa, and free ante- 

 riorly. Pallial impression with a slight posterior sinus. 



Ex. T. fragilis, H. and A. Adams, pi. 97, fig. 3, 3, a. 



The curious little shell on which this genus is founded 

 was discovered buried in sand, in a burrow in a large Spon- 

 dylus from Mazatlan, by Mr. E. W. Tyler, to whom we have 

 dedicated the genus. The calcareous lamella is connected 

 with the interior of the valves by means of vertical plates, 

 which, being produced on each side, cause the lamella to 

 assume a dentate appearance. 



Genus NEiERA,' Gray. 



Siphons short, united, the orifices of both with a few long 

 cirrhi, the anal with a membranous valve. 



Shell globose or pyriform, more or less beaked and gaping 

 posteriorly, inequivalve, the right valve the smaller, sur- 

 face smooth, striated, or ribbed longitudinally, never punc- 

 tate, beaks strengthened internally by a rib on the poste- 



