444 Mr. H. J. Carter on Specimens 



Polytrema mesentertcum, n. sp. (PL XVIII. fig. 3, a-h.) 



(Rolled specimen.) Test massive, composed of more or less 

 erect, thick, meandering laminse united mesenterically. Con- 

 sistence stony. Colour pinkish red (PL XVTII. tig. 3, 3 a). 

 Surface [i.e. vertical sides of lamina) uniformly dimpled and 

 foraminated (fig.3£,/z); margin worn away by attrition (fig. 3, c). 

 Internal structure of lamina consisting of cells or chambers of 

 different sizes, the largest and most regularly formed of which 

 are arranged cortically, where their outer wall is pierced by 

 the pore-tubulation, whose apertures on the surface are the 

 foramina before mentioned (fig. 3, c, d, e e) ; opening inter- 

 nally into a cancellated axiai structure, whose cavities, com- 

 municating freely with each other, finally open at the margin 

 in a corresponding position with respect to the cortical cham- 

 bers (fig. 3, d, g) ; thus presenting a more compact structure 

 between the layers of the latter, which, in the absence of the 

 real margin now worn away, presents the appearance of a 

 stony polyzoarium of a similar form. Fragments of sponge- 

 spicules incorporated here and there with the substance of the 

 test (fig. 3, d } i). Size of specimen, which is now rounded by 

 attrition, subelliptical and compressed vertically, 3-4th inch by 

 7-1 2th inch in its greatest horizontal diameters, and about 

 l-4th inch thick; lamina l-16th inch in transverse diameter. 



Hab. Marine. 



Loc? 



Obs. Although none of the exposed chambers at present 

 contains any sponge-spicules (probably from their having been 

 washed out), and the structure of the margin is worn away, 

 yet by the presence of fragments of sponge-spicules incorpo- 

 rated with the substance of the test, and the character of the 

 structure of the lamina internally, there can be very little 

 doubt that when in a perfect condition it closely resembled 

 Polytrema cylindricum in these respects. Still its peculiar 

 form and size not only claim for it a separate designation, but 

 the latter precedence also of all other species of Polytrema that 

 have been publicly noticed. Comparing its form with that of 

 the figure of Theone, Lamouroux (Jurassic fossil from Caen), 

 given by De Blainville, Man. d'Actinol. Atlas, pi. lxvii. 

 fig. 2, one cannot help being struck with their great resem- 

 blance. In many respects also it closely resembles the mesen- 

 teric forms of several exotic Polyzoaria. 



Genus Gypsina. 



Gen. char. Massive, sessile or free, incrusting indefinitely 

 or circumscribed and defined ; surface uniformly tessellated 



