144 Cincinnati Society of Natural History. 



varies from eight to twelve, according as there is a greater or less 

 development of .the interstitial cells. Longitudinal and tangential 

 sections give the usual characters of Heteropora, excepting that no 

 traces of connecting foramina, nor of tk radiating spines," can be de- 

 tected in my specimens. Diaphragms are usually wanting; occasion- 

 ally a few very delicate ones may be detected in the peripheral region. 



Formation and locality: Cretaceous strata (probably of the Niobrara 

 Group). Pulaski county, Arkansas. 



The third species of the Arkansas Heteropora^ I will also here pro- 

 visionally characterize under the name of 



Heteropora attenuata, n. sp. (Plate VI., fig. 12.) 



Externally the zoarium differs from that of H. consimilis, in being 

 more slender, the branches having a diameter of only about .05 inch. 

 The proper zooecia have their margins somewhat raised, and are some- 

 times arranged in transverse or oblique series. The interstitial cells 

 are small, and usually a single transverse series is developed be- 

 tween the transverse rows of the true cells : or sometimes a few of 

 the true cells may be surrounded bv a single series of interstitial cells. 

 Longitudinal sections show that the tubes in the axial or " immature" 

 region are of one kind only, and that they are thin- walled, and without 

 diaphragms. In the " mature" or cortical region the walls of the tubes 

 become thickened, and the interstitial tubes are developed. Between 

 the upper wall of each true cell-tube, and the lower wall of the inter- 

 stitial tube, may be detected, almost invariably, a lighter-colored streak, 

 indicating the presence of a " spiniform tube,'' between the upper side 

 of the cell-aperture, and the interstitial cells. Not having been suc- 

 cessful in making a good tangential section of this small form, I am 

 unable to say positively whether or not spiniform tubuli are developed 

 in other positions, but for reasons I am inclined to believe they are. 



Formation and locality : Same as the preceding. 



Having now briefly considered the principal structural characters of 

 several species of Heteropora, we will summarize the points of differ- 

 ence and resemblance between the Heteropora on the one side, and 

 Batostomella, Leioclema, and Callopora (?) Cincinnati ens is, and other 

 members of the Monticuliporidce on the other. 



(1.) Aside from the general form of the zoarium, which is of little 

 importance in a question of this kind, we find on comparison that in 

 Heteropora and the ramose t3^pes of the Monticuliporida, the zoarium 

 is composed of slender fasciculate tubes, which are nearly vertical in 



