AND ITS EEVELATIONS. 



237 



are occupied, in the living state, by tubular prolongations of the mantle, 

 the interior of which is filled with a fluid containini; minute cells and 



Ficr. 262. 



Fig. 263. 



Fig, 264. 



Fig. 262. Horizontal Section of Shell of TerebratiCla bullata (fossil, oolite). 



Fro. 263. Ditto ^- of Megerlia lima (fossil, chalk). 



Fig. 264. Ditto '.of Spiriferina roHrata (triassic). 



granules, which, from its correspouding in appearance with the fluid 

 contained in the great sinuses of the mantle, may be considered to be 

 the animal's blood. Hence these ca3cal tubes may be inferred to possess 

 a respiratory function ; and seem to be analogous to tubes of a very 

 similar nature, which extend into the ' test' of many Tunicata from 

 their sinus-system (§ 334). In the family JRhyncJioneUidce, which is 

 represented by only two recent species (the Bh. psittacea and lih. 

 nigricans, both of which formerly ranked as Terebratulae), but which 

 contains a very large proportion of fossil Brachiopods, these canals are en- 

 tirely absent ; so that the uniformity of their presence in the Terebratulida;, 

 and of their absence in the Ehynchouellida% supplies a character of great 

 value in the discrimination of the fossil shells belonging to these two groups 

 respectively. Great caution is necessaiy, however, in applying this test ; 

 mere surface-markings cannot be relied on ; and no statement on this point 

 is worthy of reliance, which is not based on a microscopic examination of 

 thin sections of the shell. In the families Spiriferida: and StropJwnemidcp, 

 on the other hand, some species possess the perforations, whilst others are 

 destitute of them ; so that their presence or absence there only serves to 

 mark out subordinate groups. This, however, is what holds good in regard 

 to characters of almost every description, in other departments of Natural 

 History, as well as in this ; a character which is of fundamental import- 

 ance from its close relation to the general plan of organisation in one group, 

 being, from its want of constancy, of far less account in another."* 



The illustrations in the above extract will give an idea of 

 the woodcuts with which the work abounds. These are all 

 executed by Mr. Bagg ; and the whole work is got up with 

 the same care that characterizes Mr. Churchill's series of 

 manuals. 



* For a particular account of the Author's researches on this group, see 

 his memoir on the subject, forming part of the Introduction of Mr. David- 

 son's ' Monograph of the British Fossil Brachiopoda,' published by the 

 Pal^eontographical Society. 



