NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 29 



usually have a dark brown pigment. The foot retractor is weak com- 

 pared to the strong byssal retractors, of which the anterior has two sepa- 

 rate branches. The byssus consists of fine yellowish threads grown out 

 from a central stem. 



Occurrence: S. algosus is found from the shore down to a few fathoms. 

 Smaller specimens are often fastened to the holdfasts of Macrocystis 

 and other algae, but usually they seem to be found along rocky shores. 

 Some samples consist of many specimens fastened together by their byssal 

 threads. 



Distribution: According to samples studied in various collections, the 

 northernmost locality where S. algosus is found is Manta, Ecuador, and 

 the southernmost is Tumbes, Chile. It is also recorded from the Juan 

 Fernandez Islands (Hassler Expedition). 



Genus PERNA Retzius 1788 



Perna Retzius, Dissertatio historico-naturalis nova Testaceorum genera, 



1788, p. 20. 

 Syn.: Chloromya Morch 1853. 



Mytiloconcha Conrad 1862. 

 Type of genus: Perna magellanica Retzius \1^^ — Mya perna Linne 

 1758. (I have been unable to find an earlier designation; therefore Perna 

 magellanica Retzius 1788 is here designated the type of genus Perna 

 Retzius 1788.) 



Remarks: Morch (1853) himself states that his Chloromya is like Perna 

 Retzius non Adanson. It is therefore remarkable that Chloromya has 

 been in use for so many years. Unfortunately Perna has been used by 

 Bruguiere 1792 (^= Pedalion) and by H. and A. Adams 1858 as of 

 Adanson (= Modiolus). Mytiloconcha Conrad was erected for his 

 Myoconcha incurva which has a very thick anterior part due to the inter- 

 nal increase of the shell. The type species has no anterior adductor, but a 

 pitted resilial ridge, and shows no special difference from old specimens 

 of Perna. 



This genus is characterized by the pitted resilial ridge, the anterior 

 position of the foot retractor, and the missing anterior adductor. The 

 shell is more margaritaceous than the other Mytilus-Wkt genera and 

 sometimes shows a beautiful green color. 



