38 INTRODUCTION. 



external surface. These differ from those of Terebratulse, however, in not arriving at 

 that surface, and in breaking-up into minute subdivisions as they approach it (Plate V, 

 figs. 8, 12). They usually open near the internal margin of the valves, by orifices so large 

 as to be apparent to the naked eye (fig. 7) ; but nearer the central part of the valves, their 

 orifices are frequently so minute as not to be readily discernible. This is in consequence 

 of the formation of an additional lamina within the old one, and of the contraction of 

 the canals in their passage through it, as is seen in the lower part of the vertical 

 section shown in Plate V, fig. 8. The texture of the shell, at the part where the muscles 

 are attached to it, is distinctly prismatic, the prisms being arranged vertically, so that 

 their polygonal extremities alone present themselves at this part of the internal surface, 

 (Plate V, fig. 9), the openings a, a, a, of the shell-canals being seen even here. It is 

 curious to observe that this prismatic substance presents itself in the external and older 

 layers of the shell, in nearly the same situation as in the most internal or newest, (as 

 shown at a, 3, Plate V, fig. 8) ; although several intervening laminae may have been 

 formed, some of them separated from the rest by a renewal of the periostracum, and the 

 shell extended by them as well as thickened. When a portion of the shell of Crania has 

 been decalcified by dilute acid, a membranous residuum is obtained, having the caecal 

 prolongations in connection with it (Plate V, fig. 11) ; these closely resemble the cseca of 

 Terebratulae, but are less filled-up with cells. The prismatic substance to which the 

 muscles are attached, yields a definite animal basis of a cellular character (Plate V, 

 fig. 13), closely resembling that of the Margaritaceous family, but more delicate and of 

 smaller dimensions, the average diameters of the prisms being not above l-1800th of 

 an inch. 



The fossil specimens of Crania which I have examined, namely, C. antiqua from the 

 chalk, and C. antiqiiior from the Great Oolite, present the same characters, so far as the 

 state of preservation of the shell allows them to be discerned. It may be remarked 

 that from the fiUing-up of the perforations with a matrix harder than the shell, and from 

 the abrasion of the latter, the large orifices of the perforations upon the inner margin of 

 the shell are frequently marked by papillary elevations, instead of by depressions. This 

 appearance is not unfrequently presented on the surface of fossil Terebratulae. 



Genus Siphonotreta, 



This genus is remarkable for the penetration of its shell (of the intimate texture of 

 which I cannot speak, on account of the metamorphic condition of the species which I 

 have examined) by canals which have the same general arrangement with those of 

 Productidse, and which, as in that family, pass up into the spinous or verrucose out- 

 growths from the shell.^ In a vertical section which I have made of the very thin shell of 



1 See Dr. S. Kutorga's Memoir 'Ueber die Siphonotretese,' &c. ; St. Petersburg, 1848. 



