36 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Ischadium recurvus (Rafinesque) 1820 

 Text-figs. 19-20 



Mytilus recurvus Rafinesque, Ann. gen. des Sci. Physiques, vol. 5, 1820, 



p. 320. 

 Syn.: Mytilus hamatus Say 1822. 

 Holotype: ? 

 Type loc: ? 



Remarks: The posterior part of the mantle is closed by a rather long 

 septum with a long narrow median tongue, the dorsal opening rather 

 short. The shell characters are those given for the genus and this species 

 is easily recognized. 



Occurrence: Reported by Dr. Tremper as collected from Newport Bay, 

 California, one living specimen identified by Dr. Dall. It has not been 

 observed later and the record may probably be due to an error, perhaps 

 a misidentification of Arcuatula demissa (Dillwyn) (cfr. discussion in the 

 Minutes of the Conchological Club of Southern California, no 36, June, 

 1944, p. 11). 



Distribution: Newport Bay, California? Atlantic: Rhode Island to West 

 Indies, Gulf of Mexico. 



Genus HORMOMYA Morch 1853 



Text-figs. 21, 23 



Hormomya Morch, Catalogus Conchyliorum quae reliquit D. Alphonso 



d'Aguirra & Gadea, comes de Yoldi. Fasc. secundus. Acephala, 1853, 



p. 53. 

 Type of genus: Mytilus exustus Linne 1758 (subsequent designation by 



Jukes-Browne 1905). 

 Remarks: Morch gave no description of his supraspecific group, but listed 

 three catalogue numbers, of which one species, the later type of the genus, 

 was properly named. Hormomya was used earlier as a subgenus of Myt- 

 ilus because of the anteriorly placed umbones, but Jukes-Browne (1905) 

 placed it as a subgenus of Brachidontes because of the sculpture. There 

 are several other characters which separate species of Hormomya from 

 BrachidonteSj and seem to necessitate a generic separation, at least until 

 a study of many species of this group has been made. 



The type of the genus, Mytilus exustus Linne, an Atlantic species 

 ranging from North Carolina to the West Indies, has anterior umbones 

 with four or five teeth, of which the anterior ones are the strongest, and 

 a few crenulations along the anterior part of the ventral margin. The 



