of the Chalk. 89 



length, thus making each depression and elevation regularly 

 round. It is evident that a section made in any part from b to 

 b, in the plane of the surface, would be more or less of the cha- 

 racter of PL VII. fig. 3, while a section made in any part perpen- 

 dicular to that plane would resemble the upper part of fig. 4, or 

 fig. C (p. 87). 



A very careful examination and comparison of innumerable 

 entire specimens and sections, both in chalk and flint, of every 

 form and variety of Ventriculidse, early satisfied me that, though 

 contrary to every figure and description that has been published, 

 the above was the true explanation of their forms. Every sub- 

 sequent observation has confirmed this conclusion. Let the 

 reader examine PL VII. figs. 1 and 2, and fig. C (p. 87), and he 

 will see three out of many of the modes in which transverse sec- 

 tions of Ventriculidse in my possession show that the membrane 

 is folded. Let him, again, examine PI. VII. figs. 1 and 2, and he 

 will see in fig. 1 a surface perfectly plain and smooth, except so far 

 as the fibrous structure marks it \ while in figure 2 he will see the 

 lower part of the surface smooth like fig. 1, but the upper part 

 gradually passing into the mammillated character. These varie- 

 ties (never before figured) are both in my possession, and I have 

 every shade and variety from the perfectly plain to the most con- 

 voluted form of fig. A above. It will easily be conceived what 

 a variety of markings in the chalk and flint, — at first sight wholly 

 inexplicable and unconnected, — will be presented on longitudinal 

 and transverse sections of such complicated convolutions. 



Nor does such variation in external form indicate in the least 

 degree a habit or power of contractility. I have shown, by 

 analogy, that it does not necessarily do so. I will show by ac- 

 tual facts that the Ventriculidae were not contractile. Not only 

 is it an important fact that we find deeply convoluted specimens 

 as wide-open-spreading as any, while we find specimens of the 

 perfectly plain varieties as narrowly funnel-shaped as any, but we 

 find, fixed on the surfaces of specimens of every variety, Ostrese, 

 Dianchorse and other shells which are in the habit of attaching 

 themselves by peculiar processes to other bodies. In every such 

 case the shape of the attached shell departs from its ordinary form, 

 and is precisely adapted to, moulded on, that of the Ventriculite — 

 proving that it grew thereon, and thus testifying at once to the 

 firmness of the texture of the body and to its noncontractility, as 

 well as to its durability. Further, we sometimes find delicate 

 Flustrse spread over parts of Ventriculites — always, as before, 

 without trace of distortion or disturbance. But there is, if pos- 

 sible, yet stronger evidence in the fact that an entire and very 

 large group of the Ventriculidse, and those, too, the very ones 

 which are pointed out as exhibiting the most " contracted state/ 1 



Ann. «8f Mag. N. Hist. Vol. xx. 7 



