156 Cincinnati Society of Natural History. 
slightly elevated, and developed in advance of the young marginal 
cells. Surface without monticules, and covered uniformly by the 
apertures of equal sized cells, twelve or thirteen of which, are ranged 
in a series .1 of an inch in length. Interstitial cells wanting. Spini- 
form tubuli may be detected on well preserved examples by viewing 
the cells obliquely. In all cases, however, they project so little that 
they are easily overlooked. 
Longitudinal sections show that the tubes in the central portion of 
the zoarium are vertical, those nearer the margin being inclined at an 
increasing angle. Their walls, when not including one of the large 
spiniform tubuli, are moderately thin, and often somewhat flexuous. 
The epithecal membrane is very thin and generally undulated. 
Diaphragms appear to be wanting. 
Tangential sections show that, with the exception of an occasion- 
al young tube, the cells are of nearly uniform size and of one kind 
only. Between the angles of junction, the majority of which are 
occupied by the comparatively large spiniform tubuli, the walls are 
thin and appear to be amalgamated, no divisional line being visible 
between the walls of adjoining cells. 
The genus Petigopora is established for the reception of at least 
four, and probably five Lower Silurian species, only one of which, the 
Chetetes petechialis, of Nicholson, has been heretofore described. 
Two are now described for the first time, the type species P. gregaria, 
and P. asperula. The remaining species I hope to be able to describe 
at some future time. The principal characters of the genus are: (1), — 
the large and numerous spiniform tubuli; and (2), the limitation of the 
growth of the colonies to small, individualized patches, which if 
brought into contact by lateral development, do not fraternize, but 
either raise a non-poriferous epithecal barrier, or leave a narrow un- 
occupied space between them. One ofthe undescribed species occurs 
in the shale washings of the upper strata of the Cincinnati group, as 
small subglobular masses, rarely exceeding .1 of an inch in diameter. 
This peculiarity of growth is due to the fact that the colony invariably 
selects some minute fragment of a shell or other foreign body, for a 
nucleus, which is eventually entirely covered. Its spiniform tubuli are 
numerous, and being of large size they are remarkably prominent, when 
in a good state of preservation. 
Petigopora gregaria is readily distinguished from all the parasitic 
Monticuliporide of the Cincinnati group, by its non-porifercus, some- 
times concentrically striated marginal band, which is always preserved, 
