BYTHOCYTHERE. 



207 



Carapace, as seen from the side, oblong, subrhomboidal, highest in front of the middle, 

 greatest height equal to more than half the length ; anterior margin rounded, posterior 

 subtruncate, slightly beaked above ; dorsal margin highest in front of the middle, thence 

 sloping steeply in front and more gently behind, slightly sinuated near the posterior 

 extremity ; ventral margin somewhat convex. Outline, seen from above, oblong-pentagonal, 

 widest in front of the middle, thence tapering suddenly to the anterior extremity, which 

 is acuminate, more gradually to the posterior, which is broad and truncate ; sides irre- 

 gularly waved and emarginate. Surface irregularly waved and rugose in a transverse 

 direction ; alajform process not very prominent, forming an irregularly excavated angular 

 line just within the ventral margin. 



Length, -^th of an inch. 

 Distribution. Recent. — Scotland. 



Fossil. — Scotland : Duntroon, Crinan, West Tarbert, Kyles of Bute, Kilchattan, 

 Curabrae, Dumbarton, Dalmuir, Inch Lonaig. England : Bridlington. Canada. 



Genus 10 — Bythocythere, G. 0. Sars. 



Valves subequal, smooth or very sparingly sculptured, almost destitute of hairs ; thin 

 and fragile. Hinge-joint quite simple or composed of a slight bar and fin-row ; no teeth. 

 Upper antennae elongated, seven-jointed ; the second joint large and thick and bearing a 

 seta on its anterior and posterior margins ; the other joints suddenly much narrower, 

 forming a long slender lash, which bears several setse ; penultimate joint linear and 

 destitute of setae. Lower antennae tolerably robust, four-jointed ; second joint large ; 

 flagellum long, biarticulate, its last joint long and setiform. Mandibles constricted above 

 the distal extremity, strongly toothed ; palp four-jointed, bearing a well-developed bran- 

 chial plate, which is set with numerous ciliated setae. Terminal lobes of the first pair of 

 jaws very short and thick ; branchial plate large, ovate, bearing numerous marginal 

 ciliated setae, and at the base four long and deflexed simple setae. Eeet elongated, 

 terminal claw very long and slender, second and third joints bearing each a short apical 

 seta ; basal joint of the first pair furnished at the base with a small lobe, which bears 

 two very large and densely ciliated, and two smaller and simple setae. Abdomen ending 

 in a very large and acuminated process ; postabdominal lobes narrow and bearing three 

 hairs. Eyes mostly absent. 



The species grouped under this genus present two very distinct types, so far as the 

 characters of the shell are concerned ; but according to the investigations of G. 0. Sars, 

 the structure of the animal itself is the same in both types. 



