174 IOWA ACADEMY OP SCIENCES. 



the meantime it seems to me that the anatomy of a single genus 

 and a single species of that genus may not be wholly without 

 interest, especially to those who are working in the interior, 

 where the Unionidse and Cyrenidas are the only available Lim- 

 ellibranchs. 



Regarding the systematic position of Sphasrium, suffice it to 

 say that the old genus Cyclas includes the present genera 

 Sphserium and Pisidium, and th^t these, with four or more 

 other generally accepted genera, go to form the family which 

 has been variously known as Cycladse, Corbiculidse and 

 CyrenidsD. 



SHELL. 



(Pig. 2.) The shell of this species is comparatively thick, of 

 a dark horn color, frequently lighter near the margins of the 

 valves, and is composed of a rather thick bluish- white nacre, 

 covered exteriorly by epidermis. Tne lines of growth are well 

 marked. The teeth are thin lamellae, 2-2 on the right valve 

 and 1-1 on the left valve. The adductor scars, a s and j) s, are 

 quite distinct and are joined dorsally by the retractor pedis 

 scars. The pallial line is rather obscure. A large specimen 

 measures 15x12x9 mm. 



MANTLE. 



The mantle consists of two thin lobes of connective tissue 

 covered by epithelium, free at their anterior and ventral mar- 

 gins, united to form the siphons posteriorly, and continuous 

 over the back. The lobes lie closely applied to the shell nacre, 

 which is secreted by them, and are attached to the nacre at the 

 pallial line by the pallial muscles, and to the epidermis through 

 the epidermal gland, which lies in a groove in the mantle mar- 

 gin. A ridge. Fig. 3, r, extending from the ventral end of the 

 anterior adductor muscle to the branchial siphon, runs along the 

 inside of each mantle lobe near its ventral margin and serves, by 

 meeting its fellow on the opposite lobe, or sides of the foot in 

 case that organ is protruded, to close the open side of the branch- 

 ial chamber and force currents of water to enter through the 

 branchial siphon, which is protruded above the mud or sand in 

 which the animal lives. The siphons. Figs. 1 and 3, & s and 

 c s, are quite muscular and are capable of considerable protru- 

 sion. Neither one is fringed with tentacles. 



MUSCULAR SYSTEM. 



The muscular system may for convenience be classed as 

 adductors, retractors, foot muscles and mantle muscles, includ- 

 ing those of the siphons. 



