98 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Lithophaga (Alyoforceps) aristata is recorded from the Lower Mio- 

 cene, Tampa, Florida, by Dall, and from the Pleistocene, Galapagos 

 Islands, by Hertlein and Strong. 



Lithophaga (Myoforceps) aristata (Dillwyn) 1817 



Plate 10, figs. 53-54 



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



1, 1817, p. 303. 

 Syn.: Modiola caudigera Lamarck 1819, Dall 1898, 1909. 

 Lithophaga caudatus Gray 1827. 

 Dactylus carpenteri Morch 1861. 

 Lithodomus forficatus Ravenel 1861, Dall 1889. 

 Holotype: ? 

 Type loc: Senegal. 



Remarks: Generally specimens of this species are easily recognized by the 

 crossing projections of the incrustation. Sometimes the incrustation has 

 no real projections, the posterior part being truncate, and the typical 

 crossing of the posterior part of the incrustation seems to be absent ; but 

 by careful inspection, one will find a thickened ridge on each valve, gen- 

 erally on the ventral side of the right valve. If these ridges had projec- 

 tions, they would cross each other. Among the sixty samples of this species 

 from the west coast of America, only two specimens had the ridge or pro- 

 jection on the dorsal side of the right valve. The form of the shell is 

 rather variable and Carpenter (1857) has named two varieties, gracilior 

 and tumidior. The variations of the outline seem to be caused by the shell 

 being bored into or by other ecological circumstances, and so do not repre- 

 sent different populations. 



Carpenter (1857) said his largest specimen measured 1.56 inches. 

 One Hancock specimen from Ecuador (403-35) has a length of 43.2 mm. 



The posterior adductor is rather large and closer to the posterior mar- 

 gin of the valve than its own length. The anterior protractor is narrow 

 and relatively short. 



Occurrence: Nearly all localities where Lithophaga aristata has been col- 

 lected are shallow water, from shore to a depth of 5 fms. It was therefore 

 astonishing to see that living specimens were collected at a depth of 165 

 fms (529-36). This species seems to be a borer of other mollusks and 

 apparently very rarely attacks stone directly. All localities in the Han- 

 cock collections of Lithophaga aristata are within the previously known 

 range of distribution of the species. 



