222 MAZATLAN UNIVALVES 
ffab.—Pznama, rare, C. B. Adams.— Mazatlan; extremely 
rare; L’pool Col. 
Tablet 1078 contains the two smallest speciraens.—1079, the 
largest specimen. 
Gents RIMULA, Der. 
Vide A. Ad. Monogr. Rim. in Proc. Zool. Soc. 1851, p. 226 :— 
H. & A. Ad. Gen. vol. i. p. 451—Non Rimula, Lowe, Proc. 
Zool. Soc. 1854, p. 181. 
This beautiful genus is exactly intermediate between Fissu- 
rella and Emarginula; representing permanently the young 
state of Fissurella, and, apparently, in its own young state 
resembling Emarginula. Like other Oolitic forms, it has 
hitherto been found recent only in the Eastern Seas. 
281. Riaruta Mazatianica, x. s. 
R. t. parva, oblonga, compressa, conict ; albida, epidermide 
tenui albofusea induté ; apice planata, angusta ; superficie 
tenue cancellaté, cancellis quadratis ; fissuré subelongata, sub- 
quadrata, intus subovali, callosd, extus interdum viz lobata, ex 
guadrante dimidium versus tote ab apice ad marginem longitu- 
dinis continua; foramine lineis inerementi decussate apicem 
versus currente; margine tenue crenulato. 
This shell would have been taken for the young of Glyphis 
inequalis, which in general appearance it greatly resembles ; 
but that fortunately the young of both that species and G. alta 
were found, differing in the following particulars. In the 
Glyphides, the hole is close to the apex, comparatively large 
and solid, and so arranged that as the hole increases, the apex 
is eaten away. In the Rimule (of which 12 specimens were 
found of different ages) the slit occupies a portion extending 
from one-fourth to one-half of the entire length from apex to 
margin; and though the largest shell is much larger than the 
Glyphis, the apex still continues in the same relative position, 
and there is a channel running up to the vertical portion, 
shewing the lines of previous slits as in Pleurotomaria. The 
lines of growth on the young shell seem to shew that at one 
time it took the form of Emarginula, afterwards enclosing its 
slit. In this respect it is the reverse of Scissurella, which 
(as appears from specimens found in the Teneriffe dredgings) 
when young is like Rimula or Trochotoma, when adult like 
