390 The American Geologist. Deceinbcr, im'T 
this wali. The septa are at quite regular intervals and are 
convex upward. On the gastral surface they j^roject slightly 
inward, as observed, into the cloaca. 
Each of these septa represents a former apertural surface, 
and the sponge affords, thus, an interesting instance of peri-- 
odical intensity of the growth- force. Tlie walls of the sponge 
thus exposed in section are very thin, approximately uniform 
in this respect, and all are perforated with straight, simple 
and relatively large canals. Those traversing the gastral 
wall (exluilent poresj are larger than the rest and appear to 
be of uniform size. The canals perforating the septal (that 
is, apertural) and exterior walls, are the inhalent pores, and 
with this necessary interpretation the septal cavities ma}'' prop- 
erly be regarded either as chambers for the accumulation and 
discharge into the cloaca, or as true ciliary cliambers. We 
tind that, for the most part, these skeletal walls have been, 
perhaps l)y secondary changes, converted into crystalline cal- 
cite, and this change has obliterated the spicular structure, 
and in some places the perforate structure of the walls. Else- 
where, especially on the external walls, there has often been a 
deposit of adventitious calcite which, in sections, gives the 
wall an unusual thickness. 
It is to be noted that there is no breach in the continuity of 
the external Avall of any given annulation. The septal or en- 
closed portions of the wall do not meet the outer or exposed 
part as they meet the gastral wall, but the entire external wall 
is arched from the peripheral base of the chamber beneath, to 
the apertural margin of the cloaca. Reference has been made 
to the vesicular tissue formed in the intermural chambers by 
thin arched tabulse which may pass from the gastral to the 
dermal wall or, if numerous, extend from one to another. These 
laminie appear to be composed of compact librous tissue which 
shows a tendency to part along a median plane as though 
constituted of two distinct layers. Steinmann did not em- 
phasize the presence of this structvire in any of the material 
embraced by him within the family Sphcerosiphonidai, but ob- 
served it as highly developed in his genus C^-yptocoelia. 
Waagen and WentzePs view, above expressed, that the pro- 
duction of such vesicular tissue is an altogether too pro- 
gessed function for a true sponge, is a suggestion too well 
