ITS 



THE MICROSCOPE AND ITS REVELATIONS 



the animal, it is difficult to form any probable idea; but it is interesting 

 to remark (in connection with the hypothseis of a relationship between 

 Brachiopods and Polyzoa) that they seem to have their parallel in exten- 

 sions of the peri-visceral cavity of many species of Plustra, Escliara, 

 Lepralia, etc., into passages excavated in the walls of the cells of the 

 polyzoary. 



571. In the Family Rhynchonellidw, which is represented by only two 

 recent species (the Rh. psittacea and Rh. nigr icons, both formerly rank- 

 ing as Terebratulae), but which contains a very large proportion of fossil 

 Brachiopods, these canals are almost entirely absent; so that the uniform- 

 ity of their presence in the Terebratulidae, and their general absence in 

 the Rhynchonellidae, supplies a character of great value in the discrimi- 

 nation of the fossil shells belonging to these two groups respectively. 

 Great caution is necessary, 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 sec- 

 tions of the shell. In the Families Spiriferidce and Strophomenidm, on 

 the other hand, some species possess the perforations, whilst others are desti- 



Fio. 394. 



FIG. 395. 



FIG. 396. 



FIG. 394. Horizontal section of Shell of Terebratula bullata (fossil, Oolite). 

 FIG. 395. Ditto . of Meaerlia lima (fossil, Chalk). 



FIG. 396. Ditto . . of Spiriferina rostrata (Triassic). 



tute of them; so that their presence or absence there serves only 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; a character which is of fundamental importance from its close 

 relation to the general plan of organization in one group, being, from its 

 want of constancy, of far less account in another. * 



572. There is not by any means the same amount of diversity in 

 the structure of the Shell in the class of Gasteropods; a certain typical 

 plan of construction being common to by far the greater number of them. 

 The small proportion of animal matter contained in most of these shells, 

 is a very marked feature in their character; and it serves to render other 



1 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. Davidson's 

 " Monograph of the British Fossil Brachiopoda," published by the Palaeontogra- 

 phical Society. A very remarkable example of the importance of the presence or 

 absence of the perforations, in distinguishing shells whose internal structure 

 shows them to be generically different, whilst from their external conformation 

 they would be supposed to be not only generically but specifically identical, will 

 be found in the " Annals of Natural History," Ser. 3, Vol. xx. (1867), p. 68. 



