292 GENERAL SUMMARY TO 



earliest stages of the development of the shell. The largest measure 3 jq, the smallest 

 3^00 °f nn mcn m diameter. These canals are charged with cellular projections of the 

 fleshy mantle, corresponding in position to the tiny cells spread over the upper surface 

 of the delicate membranaceous covering of the animal. 



In some families, however, according to Professor King, the shell-substance consists of 

 three divisions. The innermost and middle divisions, which constitute the entire thickness of 

 the valve, are calcareous, with a prismatic or fibrous structure, while the outer division con- 

 sists of a very thin membrane. The innermost and intermediate divisions are in some families 

 traversed by minute tubular canals which pass from one surface to the other, for the most 

 part in a vertical direction, and at tolerably regular intervals, but just before terminating, 

 near the outer surface of the epidermis, their orifices suddenly become dilated, the lower half 

 of the canals being considerably smaller in diameter than the upper half. The canals are 

 occupied by caecal processes proceeding from the mantle, or the fleshy covering of the 

 animal. Their functional nature is, according to Carpenter, branchial or subservient to 

 respiration ; but, as observed by Professor King, the outer epidermis, which covers the 

 expanded terminations, would seemingly prevent any communication between the sur- 

 rounding sea-water and the mantle, so that it may be questioned whether they are at all 

 connected with the respiratory function. 1 In certain genera, as Hkynchonella, there are 



in this part, the youngest of the shell, that the formation of the tubular canals commences. Not a trace 

 of them are to be seen on the posterior portion." 



1 In his valuable memoir on the ' Histology of the Test of the Class Palliobranchiata,' Prof. W. King 

 said that " the innermost division, that is, the one forming the inner surface of the valves, and nearly their 

 entire substance, is calcareous, and has a prismatic or coarsely fibrous structure, as was first made known 

 by M. A. d'Archiac and M. A. d'Orbigny in 1847 ('Mem. Soc. Gre*ol. de France,' 2nd ser., vol. ii, 

 part ii, and 'Annales des Sciences Nat., Zoologie,' 3rd series, vol. viii, pp. 255 — 257). Its component 

 prisms, which are polygonal and often compressed, lie parallel to one another, and more or less so to both 

 surfaces of the valves, sometimes appearing to form laminae ; but before terminating, which is usually 

 at both bounding planes of this division, they take an outward curve, and their terminations crop out on 

 the inside of the shell, either perpendicularly or obliquely to it ; in the former case the terminations give 

 this surface a reticulated aspect, and in the latter they overlap one another, thereby giving it, as Dr. 

 Carpenter correctly expresses, ' an imbricated appearance.' When a portion of a Terebratula shell in a 

 fresh condition is very carefully decalcified (an operation in which my colleague, Dr. llowney, has 

 succeeded remarkably well with a fragment of Megerlea truncata) the entire thickness of the present 

 division is found to be traversed by innumerable delicate ' films ' of animal matter, evidently the ' base- 

 ment membrane' detected by Dr. Bowerbank in the shells of other molluscs. These films, hitherto only 

 seen in the Ancylobranchs by the late Prof. Quekett (see ' Histological Catalogue,' 1850), at least, as far 

 as I can ascertain, appear to both Dr. Rowney and myself to be made up of confluent membranous 

 sheaths, which enclose the shell-prisms ; and we are of opinion that the termination of the sheaths, which 

 likewise crop out on the plane corresponding to the inner surface of the valves in the manner of the 

 prisms, form a sort of reticulated or imbricated lining to the said surface. In fossil shells that have lost 

 their basement membrane the individuality of the prisms is more or less obvious ; specimens of a Jurassic 

 Rhynchonella before me have the prisms so slightly coherent as to be separated easily with a camel-hair 

 pencil. The outer division (b, woodcut, p. 293), which was first discovered by Dr. Carpenter, consists of a very 

 thin membrane, corresponding to the epidermis or periostracum of other molluscous classes. The inter- 



