NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 47 



for this species and not for the Hormomya. It is closely related to Brachi- 

 dontes playasensis and Brachidontes houstonius and perhaps the three 

 species are local forms of one more variable species. 

 Occurrence: Stated to be common at the type locality, apparently inter- 

 tidal. 

 Distribution: Puntarenas and Puerto Parker, Costa Rica. 



Brachidontes houstonius Bartsch and Rehder 1939 

 Plate 3, fig. 15 



Brachidontes multiformis houstonius Bartsch and Rehder, Smithsonian 

 Miscellaneous Collections, vol. 98, no. 10, 1939, p. 14, PI. 4, figs. 4-7. 

 Holotype: United States National Museum. No. 472858. 

 Type loc: Bahia de Sulivan, Isla Santiago (James Island), Galapagos 



Islands; shore. 

 Remarks: This species was described as a finer sculptured subspecies of 

 Brachidontes multiformis, assuming that the heavily ribbed specimens 

 from Central America (here considered to be forms of Hormomya adam- 

 siana (Dunker)) represented Carpenter's species. B. houstonius is very 

 like B. puntarenensis from Costa Rica and might w^ell be considered a 

 smaller race of that species. The material studied is too small to allov*^ a 

 settlement of this question. 

 Occurrence: Intertidal on rocky shore. 

 Distribution: Galapagos Islands. 



Genus MYTELLA new genus 



Diagnosis: Shell of varying outline from mytiliform through modioliform 

 to very elongate. Umbones subterminal or nearly terminal. Anterior mar- 

 gin smooth or furnished with three or four teeth, and the lunule radiately 

 sculptured, the sculpture consisting of fine or coarse concentric lines. The 

 dorsal part usually greenish and the ventral part yellowish or brown, with 

 the greenish color sometimes concealed by darker color. The resilial ridge 

 pitted like Mytilus s. s., but sometimes so narrow that the pittings are 

 obsolete. Anterior adductor rather large, anterior retractor placed before 

 or behind the umbonal cavity, and a distinct small scar seen between the 

 anterior retractor and adductor scars. Posterior adductor confluent with 

 the posterior retractor. The two posterior retractors widely separated, but 

 the scars usually continuous; a small scar below the adductor, made by 

 the mantle muscles. The posterior part of the mantle furnished with 

 branching tentacles ; the dorsal opening with smooth margins. 



