566 



SHELL. 



which connect these plates. The Sep'wstaire 

 havin-; been formerly described in some 

 detaif (vol. i., p. 54-6), it will only be re- 

 quisite here to mention, that the calcified 

 layers which alternate with horny membranes 

 to form the shallow cone or cup, exhibit a 

 distinct cellular structure, when the section 

 is made sufficiently thin ; and that indications 

 of a similar structure may also be perceived 

 in the delicate and fragile plates which are 

 arranged obliquely upon one another in the 

 hollow of this cup. Few of the numerous 

 fossil shells referable to this class have yet 

 been examined ; it may, however, be stated 

 as an interesting result of microscopic ob- 

 servation, that the " spathose guard" of the 

 Belemnite is thereby proved to be composed 

 of long prismatic cells, radiating from the 

 centre to the circumference; closely re- 

 sembling in their general arrangement those 

 of the massive tube of Septaria g/gnntea, the 

 great sand-boring Teredo of Sumatra. 



The structure of the shells of the testa- 

 ceous Annelida, and of the pedunculate Cirrlw- 

 poda, does not essentially differ from that 

 of Mollusca; but in most of the sessile Cir- 

 rhopods, such as the common Balanus, we 

 find a cancellated structure or diploe in- 

 tervening between the inner and outer plates 

 of the shell (vol. i., p. 685). A less regular 



Fig. 421. 



verse partitions (fig. 422.). The cancelli are 

 frequently occupied by calcareous infiltra- 



Fig. 422. 



Cancellated structure from shell of Hippurite, as seen 

 in transverse section. Magnified 5 diameters. 



diploe has been described by Mr. J. E. Gray* 

 as existing between the laminae of Ostren 

 piirpurca ; but in no other shells of existing 

 Mollusca has any approach to it been yet 

 discovered. A very regular cancellated 

 structure, however, is exhibited in the singular 

 extinct group of Rud'mtcs, where it makes up 

 nearly the entire thickness of the shell (fig. 

 421.). The cancelli are usually short hexa- 

 gonal prisms, terminated at each end by a 

 flat partition ; consequently, a section taken in 

 one direction (jig. 421.) will exhibit the walls 

 of the chambers disposed in a hexagonal net- 

 work ; whilst a section that passes at right 

 angles to this will bring into view the trans- 



* Mngazine of Zoology and Botany, vol. ii. p. 228. 



Cancellated structure from the shell of Hippurite, as 

 seen in vertical section. Magnified 5 diameters. 



tion ; which might lead to the belief that, 

 like the cells of the Pinna, they were so 

 consolidated in the living state. But they 

 are also to be met with entirely empty, or 

 with their walls merely lined by calcareous 

 crystals ; so that there can be no doubt that 

 they were originally hollow. The presence 

 of this structure assists in determining the 

 zoological position of the curious group in 

 question, which many considerations would 

 lead us to regard as having been interme- 

 diate between the Bivalve Mollusca and the 

 sessile Cirrhopoda. And it may be added 

 that, by the same evidence, the place of 

 the curious Plewrorhyncus fiibernicus, a fossil 

 which has been assigned to a different tribe 

 by almost every naturalist who has examined 

 it, would unhesitatingly be determined as 

 amongst the Rudistes. 



Echinodermata. The structure of the 

 skeleton in this class is entirely different 

 from that which we have found to be cha- 

 racteristic of the Mollusca ; whilst, in its 

 essential features, it presents a remarkable 

 uniformity throughout the various members 

 of the group. The general arrangement of 

 its components is the same, for example, 

 in the firm plates which make up the testa of 

 the Echinida, in the joints of the stems and 

 branches of the Crinoidca, and in the scattered 

 calcareous deposits which are met with in the 

 integuments and in the tentacula of the Holo- 

 thurida. 



The elementary structure of the skeleton 

 of the Echinodermata may be described as 

 a net-work, composed of calcareous and 

 animal matter intimately united ; the former, 

 however, being greatly predominant. In this 

 net-work, the interspaces or areolcs, and the 

 solid structure which surrounds them, may 

 bear an extremely variable proportion to one 

 another ; so that, in two masses of equal 

 size, the one or the other may greatly pre- 



