NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 79 



specimens, while only a few seem to have been taken alive, in depths 



ranging from 16 to 50 fms. Including the valves, of which many are 



fresh, with the periostracum, the distribution is extended very far south 



in comparison to what was hitherto known. 



Distribution : Monterey, California, south to Bahia de la Independencia, 



Peru. 



Gregariella denticulata (Dall) 1871 



Modiolaria denticulata Dall, Amer. Jour. Conch., vol. 7, 1871, p. 154. 



Holotype: U. S. National Museum. 



Type loc.: Acapuico, Mexico. 



Remarks: As discussed previously, this species is at present impossible to 



interpret and is listed here in the hope that future students may be able 



to solve the puzzle. Dr. Rehder has kindly informed me that the type is 



a Gregariella. It might therefore be a synonym of Gregariella chenui 



(Recluz). 



Distribution : Acapuico, Mexico. 



Genus GRENELLA Brown 1827 

 Text-fig. 69 



Crenella Brown, Illustrations of the Conchology of Great Britain and 



Ireland, 1827, PI. 31, f^gs. 12-14. 

 Syn.: Stalagmium Conrad 1833. 

 Myoparo Lea 1833. 

 Nuculocardia Orbigny 1845. 

 Type of genus: Mytilus decussatus Montagu 1808 (monotypy). 

 Remarks: This genus comprises small inflated species with radiating 

 sculpture, a thickened and striated hinge, a sunken ligament, and a finely 

 striated and thickened dorsal margin. Otherwise the margins are crenu- 

 lated corresponding to the sculpture. The foot (text-fig. 69) is long, 

 slender, with a thick apex. The posterior retractor muscles are relatively 

 strong and fastened to the valves in connection with the posterior adduc- 

 tor. The anterior retractor could not be observed and must be very weak. 



The sculpture of typical species of Crenella can be separated into three 

 areas: the anterior and the posterior areas have more or less unilateral 

 bifurcation, the middle area has undivided or bifurcate sculpture. 



There seem to be only two species on the west coast of America, both 

 extremely variable in outline and in the characters of the hinge, which 

 change during the growth of the specimens (Bernard, 1898). The north- 

 ern species, which has been named Crenella decussata (Montagu), is 



