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Zoology. 131 
Saxicava rugosa is also furnished with a rasping surface covered with 
siliceous particles. This surface, however, in this species is formed 
entirely of the anterior portion of the mantle, the margins of which be- 
ing united are much thickened, forming a sort of cushion capable of 
considerable protrusion at the will of the animal. The foot is small, 
and passing through a much-constricted orifice, gives origin to a byssus, 
which anchors the shell close to the base of the excavation, and thus 
holds the rubbing apparatus in immediate contact with the part to be 
excavated, 
58. The Animal of the Spirula; by J. E. Gray, (Mag. Nat. Hist., 
xv, 1845, p. 257.)—Mr. Gray states that the animal of the Spirula has 
hitherto been known only from a figure in the atlas of Peron and Le- 
sueur. He concludes from his examinations of a specimen in alcohol, 
(of which he gives figures,) that they resemble closely the cuttle-fish in 
external form, and will constitute a group of the Decapodous Cephalo- 
pods, forming a passage to the Octapodous ; for like the latter, they are 
without a dorsal fin, and have a regularly chambered shell. The shell 
is enclosed with but a small part exposed, and is placed on the dorsal 
surface of the body with the spine bent towards the ventral sides. 
“The examination of this animal confirms me in the opinion, which 
I expréssed in the Synopsis of the British Museum, (1840, p. 149,) that 
the Ammonites, from their texture and the small size of the last chamber, 
are internal shells, and should be arranged with the Decapodous Cepha- 
lopods, being chiefly distinguished from the Spirulze by the siphon, al- 
ways on the dorsal margin of the whorls, and the septa being foliated 
on the edges.” | 
59. Adaptation of the Eye to Distinct Vision at Different Distan- 
ces; by Prof. Forses, (Trans: Roy. Soc. Ed., xvi, 1-)—The crystalline 
lens,—for example, that of the ox,—is composed of a nearly spherical 
nucleus of compact, comparatively dense matter, of a hard pasty con- 
sistence ; this nucleus gradually, yet rapidly passes into the gelatinous 
envelope, of a lenticular form, which has far less consistence and less 
resistance to external pressure, than the central spherule. Prof. Forses 
thence suggests that any uniform pressure applied to the lens, such as 
might be communicated by the external muscles of the eye to the en- 
tire eye-bali, and propagated by hydrostatic pressure through the hu- © 
mors, would tend to make the exceedingly flattened ellipsoid of the eye : 
approach in figure to the dense spheroidal nucleus; the obvious effect: 
of this would be, to increase the curvature of the lens without chang- 
ing its position, and thus render the rays from a near object more con- 
vergent. The effect probably proceeds from a simultaneous effort of 
the four recti muscles drawing the eye back in its socket. = 
