48 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 20 



Type of genus: Modiola guyanensis Lamarck 1819. 

 Remarks: Though one of the species tentatively included in this new 

 genus is mytiliform as to teeth, there are several distinct characters which 

 easily separate the species of this genus from other mytilids. The green 

 color, the pitted resilial ridge, and the rounded anterior retractor scar 

 with the distinct scar below, are the most important shell characters; 

 while the tentacles on the posterior pallial margin and the widely sepa- 

 rated posterior retractor muscles are important characters of the soft 

 parts. Two species, Mytella falcata (Orbigny) and Mytella guyanensis 

 (Lamarck) , live on both sides of Central America, while Mytella speciosa 

 (Reeve) is restricted to the Pacific, and Modiola papyria Conrad, which 

 perhaps should be placed in this group, is found on the Atlantic side only. 

 The fossil species named inezensis by Conrad and the more northern 

 restorationis van Winkle seem to belong to this genus also. 

 Key to the west American species: 

 1. Mytiliform with nearly anterior umbones 

 and usually with distinct teeth and radi- 



ately sculptured lunule falcata (Orbigny) 



Modioliform with smooth anterior mar- 

 gins and not terminal umbones 2 



Fig. 35. Mytella falcata (Orbigny). Laguna de Chacahua, Oaxaca, 

 Mexico. Anterior retractor behind the umbo, a small scar 

 just above the anterior adductor, distinct and apparently 

 continuous scars of the posterior retractors. 



Fig. 36. Mytella guyanensis (Lamarck). San Felipe, Gulf of Cali- 

 fornia. Anterior retractor in umbonal cavity, a small scar 

 just below the retractor scar, narrow and apparently con- 

 tinuous scars of the posterior retractors. 



Fig. 37. Mytella speciosa (Dunker). Bahia de la Magdalena, Baja 

 California. Anterior retractor before the umbo, a small 

 scar between the retractor and the adductor scars, posterior 

 retractor and adductor scars weak. 



Fig. 38. Mytella falcata (Orbigny). Laguna de Chacahua, Oaxaca, 

 Mexico. Posterior part of the mantle with papillae. 



Fig. 39. Mytella falcata (Orbigny). Laguna de Chacahua, Oaxaca, 

 Mexico, The muscles of the foot and byssus, anterior re- 

 tractor slencler, posterior retractors of the foot divided into 

 two branches. 



Fig. 40. Mytella falcata (Orbigny). Laguna de Chacahua, Oaxaca, 

 Mexico. Enlarged part of the papillae of the mantle. 



Fig. 41. Mytella guyanensis (Lamarck). San Felipe, Gulf of Cali- 

 fornia. Posterior part of the mantle with papillae. 



Fig. 42. Mytella guyanensis (Lamarck). San Felipe, Gulf of Cali- 

 fornia. Enlarged part of the papillae of the mantle from 

 preserved specimen. 



Fig. 43. Mytella guyanensis (Lamarck). San Felipe, Gulf of Cali- 

 fornia. Enlarged part of the papillae of the mantle drawn 

 from a living specimen. 



