56 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



gins are smooth without papillae or tentacles, and the outline is modioli- 

 form. Characters separating Arcuatula from Modiolus are the radial 

 sculpture ; the distinct radial folds on the lunule which make the anterior 

 margin crenulated ; the shallow form ; and the weak nymphae and the 

 relatively light ligament. 



It is difficult, without a close examination of the various Pacific species 

 assigned to Arcuatula, to decide if they should be referred to this genus 

 or not. 



Arcuatula demissa (Dillwyn) 1817 

 Plate 9, fig. 47 ; text-fig. 46 



Mytilus demissus Dillwyn, Descriptive Catalogue of Recent Shells, vol. 



1, 1817, p. 314. 

 Syn.: Modiola plicatula Lamarck 1819. 

 Modiola semicostata Conrad 1837. 

 Halo type: ? 

 Type he: Carolina. 



Remarks: This species is easily recognized by the radial sculpture and 

 Modiolus-\\ke form. The anterior retractor has a small ventral branch ; 

 otherwise the muscle scars are very like those found in Modiolus. A good 

 figure of a specimen from San Francisco Bay is given by Fitch (1953). 

 Occurrence: Reported from San Francisco Bay, introduced from the 

 east coast with seed oysters. It is common on the mud flats between San 

 Mateo and San Francisco (cfr. Hanna 1921). Mr. L. C. Bessom, Los 

 Angeles, has found this species living intertidally, but near the high tide 

 level, in the upper part of Newport Bay. One specimen has a length 

 of 90 mm. 



Distribution: California, San Francisco Bay and Newport Bay (intro- 

 duced). Atlantic. Gulf of St. Lawrence to Florida. 



Genus MODIOLUS Lamarck 1799 



Modiolus Lamarck, Mem. Soc. Hist. Nat. Paris, 1799, p. 87. 

 Syn.: Volsella Scopoli 1777. 



Modiola Lamarck 1801. 



Perna H. and A. Adams 1858. 



Eumodiolus Ihering 1900. 

 Type of genus: Mytilus juodiolus Linne 1758 (subsequent designation 



by Gray 1847.) 

 Remarks: There have been, and still are, different opinions as to the 

 validity of Scopoli's genus because he described M. modiolus Linne as 



