DR. G. J. HINDE ON RECEPTACULITIDA. 825 
mentally composed of modified extensions of the basal ends of the 
vertical rays of the spicules. instead of tapering to a pointed 
vxtremity as in Jschadites and Acanthochonia, the vertical rays, in 
this species of Receptaculites, continue cylindrical to near their basal 
extremities, and then abruptly expand into horizontal plates. These 
plates have four straight sides, but at each of the corners there is 
a semicircular or semielliptical vertical hollow. Hach plate appears 
also to be traversed by four horizontal canals, which radiate from 
the centre, where they are in connection with the canal of the 
vertical ray. The inner surface of the plate is flat; the upper 
surface, or that which is exposed in the cup or disk, is oftentimes 
convex and deeply ridged and furrowed (Pl. XX XVII. figs. 3 c-g). 
These plates are intimately united together so as to form a continuous 
inner or upper layer. The delimitations of the separate plates in this 
layer are not always preserved; in many specimens they appear to 
have been completely obliterated, and the layer resembles a continuous 
plate with numerous cylindrical or elliptical canals which penetrate 
through it at right angles. These canals are formed by the apposi- 
tion of the vertical hollows in the corners of the plates of which it 
is primarily built up. On the inner side of the plate, facing the 
interior cavity, the canal-apertures are evenly circular or elliptical, 
but on the outer surface they frequently enlarge and extend into 
irregular open channels, which run between the canals and across 
the intermediate surface. The horizontal canals in the substance of 
each plate also appear, when the plates are amalgamated together 
in one layer, to intercommunicate together. This inner layer, 
though primarily formed of plates developed from the basal ends of 
the spicules, appears, when complete, to form an independent 
membrane, and to be distinct from the vertical rays, which readily 
break off at their junction with its inner surface. It is thus 
entirely different from the spicular plates which compose the outer or 
under surface of the organism, which are clearly distinct from each 
other and form the head of each separate spicule. The inner 
surface of a silicified example, when freed from the matrix, shows 
indeed, at a superficial glance, an apparent division into component 
plates, but when closely examined this effect is seen to be produced 
by the regular lines of perforations and the deep furrows connecting 
them. 
It will be seen that my observations of the structure of the 
inner layer or endorhin of R. occidentalis agree with those of Mr. 
_ Billings. The diagram* which this author has given of the body- 
wall of Receptaculites, conveys an erroneous idea of the structure of 
the inner wall on account of the greatly exaggerated distances 
between the columns or vertical rays. In reality these are only 
about one half their own diameter apart; and if, in the limited 
- interspaces between them, room is allowed for the vertical canals, it 
will be seen that the horizontal canals are necessarily very short. 
Giimbel’s f description of the inner layer of R. Neptuni varies 
* Pal. Foss. Can. p. 382, fig. 357. 
tT Beitr. pp. 30, 31. 
