NO. 1 soot-ryen: the family iniytilidae 55 



Occurrence: M. guyanensis is found attached to stones and nearly buried 

 in muddy sand in the intertidal zone. The posterior part with the fringed 

 branchial opening is above the surface of the sand. 



Distribution: In the Pacific from the northernmost part of the Gulf of 

 California south to Payta, Peru (or Sandwich Islands, Chile, according 

 to one sample in the Museum of Comparative Zoology, Cambridge, Mass. 

 The only Chilean Sandwich Island I know is situated in Tierra del 

 Fuego, so either the label is wrong or there is another Sandwich Island 

 in northern Chile.) 



It is also recorded from Laguna de San Ignacio and Bahia de la Mag- 

 dalena, Baja California. In the Atlantic, it is found in scattered localities 

 from the Golfo de Paria, Venezuela, to Rio de Janeiro. One sample, 

 also in the Museum of Comparative Zoology, is from Puerto Rico, the 

 first record from the West Indian Islands. 



Genus ARCUATULA (Jousseaume) Lamy 1919 



Arcuatula (Jousseaume) Lamy, Paris. Bui. Mus. Nat. d'Hist. Nat., 



vol. 25, 1919, pp. 173-174. 

 Type of genus: Modiola plicatula Lamarck 1819 =: demissa (Dillwyn) 

 1817 (orig.). 



Remarks: Lamy (1919), referring to notes made by Jousseaume, says, 

 sub Modiola arcuatula Hanley: "M. le Dr. Jousseaume place le Mod. 

 arcuatula dans un nouveau genre Arcuatula, cree pour le groupe des 

 Modiola, dont presques toutes les especes sont arquees et dont la forme 

 typique est cella du M. plicatula Lamarck [= demissa Dillwyn] : les 

 coquilles de ce genre, dont les unes sont presque lisses et d'autres forte- 

 ment striees, ont le bord du ligament tres long, et Tangle forme par ce 

 bord et le posterieur est mousse et souvent arrondi ; dans le genre Brachy- 

 dontesj au contraire, le bord du ligament est court, Tangle plus saillant et 

 le bord posterieur souvent tres long et arque en dedans." The generic 

 name Arcuatula certainly was intended to be used for Modiola arcuatula 

 Hanley, but the passage given by Lamy shows that Jousseaume himself 

 chose Modiola demissa as the type of the genus. 



This common east coast shell has been referred to either Modiolus or 

 Brachidontes by various authors, and definitely needs a proper generic 

 name. The characters separating Arcuatula from Brachidontes are many, 

 of which the lack of toothlike crenulations behind the ligament is the 

 most conspicuous. Most of the distinctive characters are the same as 

 those found in Modiolus. The adductor and the retractor scars are very 

 like those found in some species of Modiolus, the posterior mantle mar- 



