128 AXEL A. OLSSON 



derneath. Umbones low with the small, adjacent beaks placed a short dis- 

 tance behind the anterior end. Anterior side is short, not auriculate, its 

 margins thickened. Dorsal margin carrying the ligament scar is straight, 

 the more posterior portion of the dorsal margin longer, descending at an 

 angle of about 30 degrees. Umbonal slope well rounded, its axis straight 

 and nearly as long as the length of the shell itself, the posterior end narrowly 

 rounded. There is a slight inflexion from the beak to the ventral margin. 

 Interior of valves nacreous and of a dull lead-white color. 



Length 93 mm., height 53 mm., semidiameter IS mm. (a right valve). 



Resembles Modiolus rectus (Conrad) of the California Coast but has 

 a straighter shell and a shorter, nonauriculate anterior side. 



Range — Northern Peru. Peru: Salinas on the Bay of Sechura to the 

 northeast of Bayovar. Paita. 



Genus MUSCULUS Roeding, 1798 

 {Modiolaria Beck, 1838; Lanistes (Humph.), Swainson, 1840). 



Type species by subsequent designation, Iredale, 1915, Mytilus discors 

 Linne. 



Musculus Roeding has been considered preoccupied by MuscuLus 

 Martyn, 1787 but Martyn's names are unavailable. 



Shells comparatively small, rhomboidal, modioliform, inflated, moder- 

 ately thin and with a pearly interior. Umbones prominent with the small 

 beaks not quite terminal. Sculpture of surface divided into three areas, 

 the anterior and posterior portions are marked with small radial threadlets, 

 the middle zone smooth. Ligament subinternal and attached to a narrow, 

 inset area under the hinge margin. Hinge edentulous. 



Subgenus GEEGARIELLA Monterosato, 1884 



(Botulina Dall, 1889; Trichomus cuius Iredale, 1924: Tibialectus Ire- 

 dale, 1939). 



Type species by subsequent designation, Crosse, 1885, Modiolus sulcatus 

 Risso, 1826 (not of Lamarck, 1805, 1819) = Modiolus barbatellus Can- 

 traine, 1835 = Modiolus opifex Say, 1825. 



Shell modioliform, straight, convex, the beaks almost terminal, the an- 

 terior margin below not passing beyond them or but slightly. Posterior side 

 long, often somewhat attenuated at the end, the umbonal slope narrowly 

 vaulted to subangulated, with the radial riblets divaricating along it. Sur- 

 face marked with fine, radial riblets (decussated on the dorsal side) over 

 the whole posterior and mid-posterior slopes, also the anterior slope but 

 with the anterior-middle-ventral zone smooth; the whole covered by a 

 brown, skinlike periostracum, bearing a mat of hairs mixed with grains of 

 sand along the posterior-umbonal slope, often terminating in a tuft at the 

 end. Inner margins of valves finely crenuated by the ends of the radial 

 riblets, except along the anterior-middle portion. Resilifer narrow and linear, 

 a trifle less than half the length of the dorsal margin and at the forward 

 end passing between the reflexed end of the anterior margin and the beaks. 



