96 Mr. Toulmin Smith on the Ventriculida? 



chalk, the place of the fibre left hollow and the intervals filled with 

 chalk) of the delicate membrane which underlies the polyp-skin. 



The polyp-skin itself is of extreme tenuity, and differs alto- 

 gether in structure from the internal parts. It is not fibrous, but 

 of a uniformly close texture which yields to the highest powers 

 of the microscope no other than a minutely granular appearance. 



It is only in very rare cases that the polyp-skin is found in 

 any degree of perfection. Careful observation and comparison 

 of an immense number of specimens, with their casts, led me to 

 infer the existence of this polyp-skin, as matter of unevadable con- 

 clusion, long before I was fortunate enough to find, as I have 

 since done, specimens in which it was so preserved as to be found 

 on the body of the fossil itself. In such cases as the latter, the 

 internal structure is always clearly seen where any parts of the 

 polyp-skin are ruptured. See PL VII. fig. 12. 



I shall presently enter on the consideration of the polyps 

 themselves and the accompanying phenomena. 



It will of course be understood that the membranes I have de- 

 scribed must, during the life of the animal, have been filled with 

 soft parts, which, with whatever minutely ramifying vessels they 

 contained, rapidly decomposed, and of which therefore 1 have as 

 yet discovered no trace. 



There existed no spiculse, siliceous or calcareous, in any of the 

 Ventriculidse ; some spiculse in the adjoining chalk or fiint may 

 sometimes deceive an inexperienced observer. 



The structure of the root differs much from that of the body. 

 Annexed to the body by short fibres clearly seen on a good sec- 

 tion, it is, like the body, arranged in squares, but those squares 

 have not all that regularity which those of the body have. They 

 are, in general, smaller in their average size than those of the 

 body, and have throughout a tendency to elongation in their 

 longitudinal direction, whence they are often narrowed on one 

 or both of their lateral planes. The fibres of the root are also 

 thicker in their longitudinal direction than in their transverse. 



It is particularly important to observe that there is no trace 

 in the roots of the octahedral structure, a fact precisely in ac- 

 cordance with the functions of those roots. The safety of the 

 animal would be more secured by the latter yielding to every 

 impulse, and waving their so delicate load from side to side, than 

 by an unbending stiffness. The octahedral structure had there- 

 fore no place in the roots. 



The integument of the root was also very different from that of 

 the body. It is impossible for a moment to confound the two. Its 

 longitudinal fibres were much thicker than the transverse ones j 

 and, there being no octahedral structure to secure an unyielding 

 exactness, it appears as if disposed in long, narrow, and not very 

 regular meshes. When encrusted with calcedony it appears not 



