14 REPORT — 1844. 



which passes in the plane of the lamina (fig. 20). From Ihis network some 

 branches proceed towards the nearer side of the section, as if to join the net- 

 work of another layer ; whilst others dip downwards, as if for a similar pur- 

 pose. The most characteristic examples of this structure which I have met 

 with are to be found in the outer yellow layer of Anomia ephippittm (fig. 4-0), 

 the external layer of Lima scabra, and in Chama Jlorida. In other in- 

 stances, the tubes run at a distance from each other obliquely through the 

 shelly layers, and they are then usually of large size. This is the case for 

 instance in Area Noce, and Pectunculus. In no cases liave I seen any such 

 variation in the size of the tubes of the same shell, as would convey the idea 

 of their resemblance to blood-vessels ; and even where a division occui-s, the 

 size of each of the branches is usually equal to that of the single trunk. 

 Sometimes these canals are quite straight, whilst in other instances they are 

 sinuous. That they are not mere channels or excavations in the shell-sub- 

 stance, is proved by the fact that they may be seen in the decalcified mem- 

 brane (fig. 41). I have frequently seen in them indications of a cellular 

 origin, as if they had been formed by the coalescence of a number of cells 

 arranged in a linear direction ; and I find that Mr. Bowerbank has come to 

 the same conclusion. 



31. The tubular structure is usually found only in the ordinary membra- 

 nous shell-substance; in fact, I have seldom observed it in the nacre, except 

 where the tubes penetrate this, to be distributed in a layer external to it, 

 as is the case, for example, in Anomia and Trigonia. I have nowhere ibund 

 it coexisting in the same shell with any great amount of prismatic cellular 

 substance ; consequently it is for the most part absent in the Margarita- 

 cecE and NayadecE, and but very slightly manifested in the true Ostracece, 

 In most of the families of Bivalves, however, in which the lobes of the 

 mantle are united, some traces of it may be detected ; though these are often 

 very scanty. There is less regularity in regard to this character, than in 

 respect to most others furnished by the microscopic examination of the 

 shell. Thus I have found a little collection of tubes in one spot of the nacre 

 of an Avicula, in no other part of which did I meet with any ; and I have 

 frequently found one species of a genus extremely tubular, whilst another, 

 closely allied to it, was almost or entirely destitute of tubes. Nevertheless, 

 in conjunction with other characters, I consider that the presence or absence 

 of this structure may often afford valuable assistance in determining the 

 position of an unknown specimen. Of this I shall presently adduce a stri- 

 king example. 



VIII. Cancellated Structure. 



32. I give this denomination to a peculiar structure closely resembling 

 the cancellated texture of bone, which is remarkably characteristic of that 

 very peculiar and perplexing group, — the Rudistes. I can scarcely de- 

 scribe this structure so well, as by comparing it with the prismatic cellular 

 structure oi Pinna and its allies, upon a large scale ; v>ith this important dif- 

 ference, however, that in this cancellated structure the prismatic cells are 

 not solid but hollow*. It is true that in many specimens of Hippurite and 

 Sphserulite, the cancelli are found to be completely filled with carbonate of 

 lime ; but there are appearances about this deposit, which lead to the belief 

 that it is the work of subsequent infiltration ; and this view is confirmed by 

 the fact, that the Rudistes of the Chalk are commonly found with their can- 

 celli empty. In what manner these minute chambers were occupied during 



* This structure has been described bv Mr. Gray in the Magazine of Zoologj' and Botanv, 

 vol. ii. p. 228. 



