mazatlan bivalves 137 



187. Aeca emaeginata, Sow. 



Troc. Zool. Sac. 1833, p. 20.— ^fiiU. S//12. Test. Viv. p. ISO — 

 Eve. Conch. Ic. pi. 4, f. 26.— Hani. Descr. Cat. p. 161, pi. 18, 

 f. m.—B. M. Cat. D'Orh. Moll. p. 81, no. 723.- C. B. Ad. 

 Pan. Shells, p. 258, no. 415. 



Although classed by Sby. among the ftquivalves, he rightly 

 describes it as inffiquivalve, the posterior ventral part consider- 

 ably overlapping, even in young shells. Umbos separated by 

 a narrow triangidar area. Ligament in very young shells only 

 adhering x^osteriorlj^ : afterwards filling a triangular space to 

 the beaks ; when adult displaying also a thin film on the 

 anterior portion. Teeth strong, not very numerous, both rows 

 forming a regidar, slightly curved line, not reaching into the 

 wing. Emargination not developed in young shells, variable 

 in adult. Verj' young shell smooth : afterwards beautifully 

 coloured with irregular dark chocolate undidating bands, and 

 an interior spot radiating from the lunbos. The smallest 

 specimen is '03 across: the largest (valve) lor\g. "5, lat. '9. 

 alt. '35. 



Hah. — Atacamas, Real Llejos, Xipixai^i, Panama ; in sandy 

 mud, 6-8 fm. ; Cuming.— Do. ; Z)'0;'i/V/«^.— Panama, very 

 rare ; C. B. Adams. — Gulf of California, Mus. Cuming. — 

 Mazatlan ; young on Spondylus calcifer, adult on Murex 

 nigritus, extremely rare ; Upool Col. 

 Tablet 640 contains 4sp. young. — 641, the largest valve. 



188, Aeca: ,jun., sp. ind. 



Comp. Byssoarca alternata, Sow. Proe. Zool. Soc. 1833, p. 17. — 

 (Area a.) C. B. Ad. Pan. Shells, p. 257, no. 413. (Hah. 

 W. Colimibia, Cuming : Panama, C. B. Adams : Mazatlan, 

 Col. Jewett.J 



Tablet 642 contains 2 small opposite valves, the largest 'Oti 

 across, remarkable for the structure of the hinge, which is 

 bounded not by one but by two lines, one of which radiates 

 from the umbo, and between ^^hich probably the ligament is 

 fixed. Outside are concentric irregular lines near the umbo, 

 after^'ards about 40 well rounded regular ribs, with equal 

 interstices and strong plications within. It is clearly a j'cung 

 shell ; too young to determine even the generic section with 

 confidence. 



Hab. — Mazatlan ; oiF Spondylus calcifer, extremely rare ; 

 JJ'pool Col. 



