NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 23 



the scar of the anterior adductor is placed between the dorsal and ventral 

 margin in the anterior angle. The mantle margins are furnished with 

 large tentacles arranged in groups, and are generally heavily pigmented. 

 Large specimens are recorded to a length of nine inches. 

 Occurrence: Mytilus calif ornianus is very common, especially along the 

 more exposed coast in the intcrtidal zone. Sometimes found living in 

 deeper water to 25 fms (Berry, 1954). 

 Distribution: Aleutian Islands south to Isla Socorro, Mexico. 



Genus CRENOMYTILUS new genus 

 Plate 2, figs. 9-10 ; text-fig. 7 



Syn.: Mytiloconcha auct. non Conrad 1862. 



Diagnosis: Shell mytiliform with terminal umbones, lunule grooved and 

 incurved, forming two to three large teeth usually obsolete in old speci- 

 mens, as there is a pronounced tendency to a growth in thickness by de- 

 positing new shell material on the inside of the valves. Margins, espe- 

 cially the anteroventral one, finely crenulated, the crenulations apparently 

 formed as a result of the building up of the crystals in the valves ; shell 

 obliquely striated, especially distinct on the ventral surface. Resilial ridge 

 compact; anterior adductor strong, showing a distinct thickened scar; 

 anterior retractor scar elongate behind umbo ; posterior adductor and 

 retractor scars continuous. The soft parts have not been studied. 

 Type of genus: Mytilus gray anus Dunker 1853. 



This group of mytiliform species is distinguished from similar forms 

 by the compact resilial ridge, the strong anterior adductor, and the fine 

 crenulation of the margins. No recent species is recorded from America, 

 but during the Tertiary this group seems to have been widely distributed 

 along the west coast of North America. These species have usually been 

 listed as Alytiloconcha Conrad, which, however, seems to be based on an 

 old, thick specimen of Alyoconcha incurva Conrad without anterior ad- 

 ductor and apparently belonging to Perna Retzius. 



The west coast Tertiary species are : Crenomytilus mathewsoni 

 (Gabb) 1866 (Oligocene) ; C. trampasensis (Clark) 1915 (Miocene) ; 

 C. coalingensis (Arnold) 1910 (Upper Miocene) ; C. kewi (Nomland) 

 1916 (Pliocene) ; and perhaps also other species listed as Mytilus. The 

 only recent species seems to be C. gray anus (Dunker) 1853 = M. dun- 

 keri Reeve 1857, from the Kuril Islands south to the Philippine Islands. 

 The name Mytilus dunkeri has erroneously been used for the form 

 of Alytilus named diegensis by Coe (1945). 



