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, 
Var iste Lok, 
_ Hab.—Mazatlan; not common; L’pool & 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 ineurved.—564, 1 large specimen, somewhat distorted. 
Tablet 565 contains a valve ‘05 long, of a oval form, with 
the concentric furrows clearly marked. 
Grenos CRENELLA, Brown. 
Crenella, Brown, 1827. —Lanistes, Swains. 1840, (non Montf.)— 
Lanistina, Crar y, 1847.—Myoparo, Lea, 1833.—Modiola, sp. 
Lam. 
172. CRENELLA coARcTATA, Dkr. 
Dunker in lit.—No. 185, 190, Mus. Cuming. 
Comp. Modiola Chenuanus, #Récl.= Mytilus C., B. MW. Cat. 
D Orb. Moll. p. 84, 754, 
Comp. 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 
_ Strie, divaricating on the diagonal angle, which in adult shells 
is clothed with a very coarse bristly epidermis. The strie 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 haying been so found in the umbilical portion of * 
