MAZATLAN BIVALVES 137 



187. ARCA EMAROINATA, Sow. 



Proc. Zool. Soc. 1833, p. 20. Mull. Si/n. Test. Viv. p. 180. 

 Eve. Conch. Ic. pi. 4, f. 2Q.Hanl. Dcscr. Cat. p. 161, pi. 18, 

 f. 60. J?. M. Cat. D'Orl. Moll p. 81, no. 723,- C. B. Ad. 

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



Although classed by Sby. among the sequivalves, he rightly 

 describes it as insequivalve, the posterior ventral part consider- 

 ably overlapping, even in young shells. Umbos separated by 

 a narrow triangular area. Ligament in very young shells only 

 adhering posteriorly : 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 regular, slightly curved line, not reaching into the 

 wing. Emargination not developed in young shells, variable 

 in adult. Very young shell smooth : afterwards beautifully 

 coloured with irregular dark chocolate undulating bands, and 

 an interior spot radiating from the umbos. The smallest 

 specimen is '03 across : the largest (valve) long. '5, lat. '9, 

 alt. "35. 



Hob. Atacamas, Real Llejos, Xipixapi, Panama ; in sandy 

 mud, 6-8 fm. ; Cuming. Do. ; D' Orbigny. Panama, very 

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

 Mazatlan ; young on Spondylus calcifer, adult on Murex 

 nigritus, extremely rare ; L'pool Col. 

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



188. ARCA , jun., sp. ind. 



Comp. Byssoarca alternata, Sow. Proc. Zool. Soc. 1833, p. 17. 



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



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



Col. Jewett.) 



Tablet 642 contains 2 small opposite valves, the largest '06 

 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 which probably the ligament is 

 fixed. Outside are concentric irregular lines near the umbo, 

 afterwards about 40 well rounded regular ribs, with equal 

 interstices and strong plications within. It is clearly a young 

 shell ; too young to determine even the generic section with 

 confidence. 

 Hob. Mazatlan ; off Spondylus calcifer, extremely rare ; 



L'pool Col. 



