86 THE CRAG POLYZOA. 



slightly rugose in parts ; whilst the upper part of the margin of the cell is not produced 

 downwards so distinctly in the form of a horse-shoe as it is in Cup. Johnsoni, whose poly- 

 zoary, again, is very conical, in which respect the species seems to agree with the 

 description of C. denticulata. 



C. denticulata of the Crag appears to attain a larger size than the recent C. Jolmsoni, 

 and it varies very much in shape, more especially in the greater or less elevation of the 

 cone, which in some cases is very much depressed, whilst in others its height equals, or 

 slightly exceeds, the diameter. But one or two peculiarities will nearly in all cases be 

 observed. The form of the disc is seldom if ever truly circular, but more or less oblong, 

 and usually showing a disposition to become angular on one or two sides. A second 

 peculiarity, which I have not noticed in the recent species, consists in the circumstance 

 that in C. denticulata the cells are often disposed to run in parallel rows, from the side of 

 which other parallel series run off at right angles ; a disposition which does not appear to 

 be manifested in the recent forms. When in tolerably perfect condition, no difficulty 

 will be experienced in identifying specimens of C. denticulata, but when they are much 

 worn, as is very frequently the case, in which state it is probably the L. alveolatus of Mr. 

 S. Wood, the identification will not be found quite so obvious. In PI. XIII, fig. 3, is a 

 representation of the appearance presented by a worn specimen, in which it will be 

 observed that the denticulate margin of the aperture is destroyed, the situation of the cell 

 being indicated by a more or less quadrangular, oblong opening, above which the remains 

 of the vibracular cavity will often be seen, represented by a shallow, cup-like depression, the 

 intervening substance being rough, and usually more or less porous. In a more advanced 

 stage of attrition, the vibracular cup is wholly removed, when the surface of the polyzoary 

 throughout a considerable extent will exhibit nothing but parallel rows of oblong, shallow 

 pits, some distance apart. In this state it would be very difficult in many cases, and in 

 some perhaps impossible, to distinguish a specimen of C. denticulata from one of C. 

 canariensis, without reference to the condition of the posterior surface, when the presence 

 or absence of the large pores, and the subdivision or not of the ridges into quadrangular 

 portions, will at once determine the species. In its worn condition, and in small frag- 

 ments, C. porosa may also be mistaken for imperfect fragments of C. denticulata, but in 

 C. porosa, the fossae left by the worn-down cells are of a pyriforra shape, and disposed 

 very distinctly in oblique lines, whilst the vibracular cup is more usually wholly obli- 

 terated, owing to the circumstance that it is, when perfect, much shallower in that species 

 than in C. denticulata or C. canariensis. The fine porosity, moreover, of the posterior 

 surface in C. porosa, affords a further mark of distinction. 



