350 AXEL A. OLSSON 



Genus TAGELUS Gray, 1847 

 Type species by original designation, Solen guineensis Chemnitz ( = 

 Solen gibbus Spengler). Recent along the east Atlantic Coast. 



Shell with elongated, solen-like valves but with the beaks near the 

 middle, the length between three and four times the height. The anterior 

 side is generally a little fuller, the posterior side often slightly depressed, 

 especially near its dorsal margin. Both ends of the valves are rounded 

 and generally with a wide open gap. Hinge weak with two small cardinal 

 teeth in the right valve and only one In the left; there are no laterals. The 

 ligament is external and attached to a high nymphal plate. Adductor scars 

 large, placed close to the dorsal margin, and connected across by the pallial 

 line bearing a large open sinus. There is sometimes a slight flexure across 

 the middle of the valves, the surface marked with lines of growth only, 

 white or colored faintly with violet or brown, sometimes rayed. The perio- 

 stracum is dark brown or straw-colored, generally coarsely wrinkled. 



This genus is generally divided into two subgenera on the presence or 

 absence of an internal rib in the umbonal cavity. This character has little 

 systematic value but Is helpful In Identification and segregation of the 

 species. 



I. Shell without an internal rib in the umbonal cavity. 



Subgenus Tagelus, s.s. 



II. Shell with a small thickened rib in the umbonal cavity, usually showing 

 as a small purple or violet streak on the surface of the umbone. 



Subgenus M esopleura 



General key to species of Tagelus 



I. Shell without a thickened rib or ray In the umbonal cavity. 



Subgenus Tagelus, s.s. 



A. Pallial sinus not reaching quite to the middle. Shell elongated. Its 

 height about a fourth of its length. Ends of the valves white or shaded 

 with violet or brown, the umbones rayed. 



T. dombeii 



B. Pallial sinus extending to the middle of the shell cavity or to a point 

 directly under the beak. Color of exterior white under an olive or straw- 

 yellow periostracum. 



a. Ratio of height to length 1 to 3. Periostracum straw-colored. 



T. affinis 



b. Ratio of 1 to 2.25. Periostracum olivaceous. 



T. peruanus 



II. Shell with a thickened rib In the umbonal cavity, generally colored 

 violet or brown and showing through the wall onto the surface of the 

 umbone as a colored ray. This internal rib is always well developed In 

 the young but may not persist throughout life. 



Subgenus Mesopleura Conrad 



C. Shell large (60 mm. or longer), thick, with a wide, flangelike wing 

 along the posterior-dorsal margin. 



T. peruvianus 



D. Shell smaller and usually of thin texture. 



c. The Internal ray Is directed backward. 



T. politus 



d. Internal ray vertical or directed forward. 



T. bourgeoisae 



