26 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



localities from this expedition are wrong, as the material had been 

 handled by incompetent nonscientific personnel. The excellent figure 

 in the plates from the Expedition leaves no doubt that this species is 

 the rather common South American species later recorded under several 

 different names. The Exploring Expedition visited Valparaiso, Chile, 

 and Callao, Peru. It therefore seems safe to make Valparaiso the type 

 locality. 



There has been much confusion about this common and variable species 

 and different authors have described the varieties several times. Some 

 names have been used in another sense than the describer intended and 

 all these names have been listed in various ways by later authors. As 

 far as it can be determined after a study of many collections, there is 

 only one species of this group along the coasts of South America. Some- 

 times the specimens are elongated, sometimes short and more inflated, 

 and small samples of each form may easily be considered to represent 

 two different species. 



The main characters separating S. algosus from other related species 

 are those as given under the genus Semimytilus. The figures of the shell 

 with muscle scars show that the anterior margin grows backward and 

 often ends in a toothlike point. The fold bordering the more dull lunule 

 is sometimes difficult to see, but with some experience this species can 

 be determined with the valves closed. Young specimens may be very 

 like young Chorornytilus palliopunctatuSj but the form and placement 

 of the anterior retractor easily separate the two species. 



The posterior part of the mantle is furnished with branched tentacles 

 and papillae of a very distinct form. The dorsal siphon is long and pro- 

 truding, with smooth margins; the septum is short. The posterior parts 



Fig. 10. Mytilus edulis Linne. Yaquina Bay, Oregon. Common 



form, posterior part of mantle. A. P.- posterior adductor; 



D.O.- Dorsal or anal opening; S.- septum connecting both 



mantle flaps in the branchial opening. 

 Fig. 11. Mytilus edulis Linne. Newport Harbor. High form {die- 



gensis Coe). Posterior part of mantle with small papillae. 

 Fig. 12. Mytilus californianus Conrad. San Pedro. Posterior part 



of mantle, a. Enlarged papillae. 

 Fig. 13. Chorornytilus palliopunctatus (Carpenter). Salina Cruz, 



Oaxaca, Mexico. Posterior part of mantle, a. Enlarged 



papillae. 

 Fig. 14. Semimytilus algosus (Gould). North Chincha Island, Peru. 



Posterior part of mantle, a. Enlarged papillae from the 



inside, b. Enlarged papillae from the outside. 

 Fig. 15. Sernunytilus algosus (Gould). North Chincha Island, 



Peru. A.R.B. - anterior retractor byssi, here divided into 



two branches; B. - byssus; F.R.-foot retractor; P. R. B. - 



posterior retractor byssi. 



