52 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



greenish hue is observable at least on the keel. The periostracum is more 

 or less shining, with a narrow dull stripe along the ligament. The interior 

 is usually dark purple. 



It is easy to confuse this species with Mytilus s. s. The best character, 

 aside from the color, is the round scar made by the anterior retractor, as 

 this scar is always elongate in Mytilus s. s. Between the anterior adductor 

 and retractor a small but distinct scar is seen. This scar seems to be made 

 by a thickening of the mantle and not by a branch of the anterior retrac- 

 tor. The posterior retractores bysii are widely separated, though the scars 

 are continuous. The foot retractor is small and attached in front of the 

 first posterior retractor. The byssus consists of numerous fine threads 

 from a main stem. 



The posterior part of the mantle margin is folded and furnished with 

 papillae or tentacles from the posteroventral corner upward to where the 

 two lobes unite. The dorsal opening has smooth, slightly protruding mar- 

 gins. The septum terminates in a triangular flap. Mantle margins and 

 septum are pigmented with dark brown in the specimens studied. The 

 labial palps are very long, reaching to the foot. 



Occurrence: As far as is known, this species lives in the mud on intertidal 

 flats and in shallow lagoons down to 6 fms. One sample was taken from 

 the bottom of a boat in the estuary of Guaya, Ecuador. The two samples 

 from the Hancock expeditions extend the distribution north to southern 

 Mexico. 



Distribution: The northernmost Pacific record is from Bahia de Petatlan, 

 Mexico. (One specimen in the San Diego Museum was found in a 

 sample of Mytella guyanensis from La Paz, Baja California, but ap- 

 parently had been misplaced.) The southernmost Pacific record seems to 

 be the estuary of Guaya, Ecuador. A sample in the California Academy 

 of Sciences is from Isla Baltra (South Seymour Island), the Galapagos 

 Islands. On the Atlantic side, Mytella falcata is recorded from the Golfo 

 de Paria, Venezuela, to Cabo San Antonio, Argentina. 



Mytella speciosa (Reeve) 1857 

 Plate 5, fig. 25 ; text-fig. 37 



Modiola speciosa Reeve, Conchologia Iconica. Modiola, 1857, PI. 7, 



Species 35. 

 Syn.: Modiolus tumbezensis Pilsbry and Olsson 1935. 

 Holotype: British Museum? 

 Type loc. : Tumbes, Peru. 



