MAZATLAN BIVALVES 123 



investigated. The N. Zealand specimens appear intermediate 

 between this and the typical form, differing from the latter in 

 being a flatter shell, with a well rounded posterior margin. 

 This shell appears to take all allowable forms except the typi- 

 cal one, the margins never being so straight and angular, and 

 the diagonal keel being less impressed. The muscular impres- 

 sions vary somewhat, the posterior adductor being generally 

 retort-shaped. The young shells display a fine olive spotting 

 on a light ground in the posterior part, which is always much 

 produced. The largest specimen (distorted) measures long. 2*5, 

 lat. 1-2. alt. 1*1. 

 Hob. Mazatlan ; not common ; L'pool 8f Havre Coll. ? New 



Zealand, Hincks. 



Tablet 559 contains 1 adolescent specimen, oval form. 

 560, 2 sp. broad, dorsal margin straight. 561, 2 do. very 

 long. 562, 2 sp. -slightly curved. 563, 2 do. ventral margin 

 much incurved. 564, 1 large specimen, somewhat distorted. 



Tablet 565 contains a valve '05 long, of a oval form, with 

 the concentric furrows clearly marked. 



G-ENtis CBEKELLA, Brown. 



Crenella, 'Brown, 1827. Lanistes, Swains. 1840, (non Monff.) 

 Lanistina, Gray, 1847. Myoparo, Lea, 1833. Modiola, sp. 

 Lam. 



172. CKENELLA COAKCTATA, DJcr. 



DunJcer in lit. JN~o. 185, 190, Mus. Cuming. 



Comp. Modiola Chenuanus, JfeW. = My tilus C., B. M. Cat. 



D'Orl. Moll. p. 84, 754. 

 Cornp. M. opifex, Say. 



Shell very variable in shape, but generally very tumid, with 

 a medial constricting line, between which and the anterior 

 part it is nearly smooth : the rest with rather fine radiating 

 strise, divaricating on the diagonal angle, which in adult shells 

 is clothed with a very coarse bristly epidermis. The strige on 

 the dorsal part, which is much hollowed by the protuberance 

 of the umbos and the diagonal angle, are somewhat decussated. 

 It appears to have the power of burrowing, like Lithophagus, 

 a specimen having been so found in the umbilical portion of 



