62 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



especially visible in the umbonal region. The hairy protuberances of the 

 periostracum and the serrations on them are somewhat variable, from 

 narrow to broader hairs with shorter or longer serrations on one side. 

 Different types may be found on the same specimen. Carpenter ( 1855-57) 

 says that very young specimens have long but not serrated hairs ; usually 

 serrated hairs are distinct even on specimens only 10 mm long. Two speci- 

 mens with a length of 12 mm, one from Pt. Fermin, the other from Cata- 

 lina Island, might be aberrant specimens of M. capax, as they have spade- 

 shaped, but smooth, periostracum hairs. 



The inside is generally reddish-violet on the posterior half. The dorsal 

 mantle opening is rather large, elongate, usually with the rectum visible. 

 The mantle edges of the branchial part are thin and the septum short, so 

 the connection between the branchial and anal cavities is wide open. 

 Occurrence: Intertidal on rocks or boulders, often in mud down to 

 25 fms. 

 Distribution : Santa Cruz, California, to Payta, Peru ; Galapagos Islands. 



Modiolus fornicatus (Carpenter) 1865 

 Plate 6, fig. 26 



Modiola fornicata Carpenter, Ann. and Mag. Nat. Hist., sen 3, vol. 15, 



1865, p. 178. 

 Holotype: In the collection of Mrs. Boyce, Utica, N. Y. 

 Type he: Santa Barbara, California. 



Remarks: This species has been misinterpreted several times and the name 

 fornicata has been used for specimens of M. sacculifer (Berry) . M. forni- 

 catus is easily recognized in typical specimens by the very inflated and 

 curved oldest part of the shell, with the umbones protruding beyond the 

 anterior margin. Smaller, less inflated specimens with less curved and 

 protruding umbones, seem to be very like the young of other species of 

 Modiolus. Good representative material of all sizes must be studied be- 

 fore M. fornicatus can be correctly described. 



Large specimens between 1 1 mm and 35 mm have thick valves with a 

 violet line inside, and a shining brown periostracum with remains of 

 smooth hairs on the posterior part. The ligament is relatively short, the 

 anterior part concealed between the umbo and the anterior margin; the 

 anterior adductor is deepset, distant from the anteroventral margin ; and 

 the anterior retractor is fastened in the deep umbonal cavity. 

 Occurrence: Very little is known about the habitat of this species, as it 

 has been confused with M. sacculifer (Berry). 



Distribution: Trinidad, Humboldt County, California, to San Pedro 

 and Cortez Bank, California. 



