THE FvVUNA OF THE ST. JOHN (iEOUP. 47 



Fiuding that the three classes of univalve molluscs agree as to the sides of the shell, 

 wliiih may be regarded as dorsal and ventral, we may well infer that the Hyolithoid 

 forms, which exhibit aihnities to the three, are likely to be in harmony with them in the 

 matter of the dorsal and ventral sides. 



Habitat. Whatever may have been the sphere of existence of the later species of 

 Hyolithes there is no doubt that the early Cambrian forms of this type lived under condi- 

 tions quite at variance with the habitat and mode of life of the modern Pteropods. It is 

 obvious that all the thecoid shells of the St. John group belong to one class of molluscs, 

 but of these shells some at least had strictly littoral habits. This was more especially the 

 rase with those of the genus Diplollieca (described below.) It is in this genus that we 

 liud the internal skeleton most complicated in its structure, and at the same time firmest 

 to resist pressure from without. The shells of this genus were among the first of the 

 oncephalous mollusca to invade the St. John Basin ; for they are found buried in the sand 

 casts which fill the worm burrows, that are so common in certain parts of the sandstones 

 of Division l.è; they are also to be found in the phosi^hatic nodules whit-h are scattered 

 over the surfaces of this sandstone, and solitary shells are found sparsely spread on the 

 sandy, ripple-marked layers of this rock. Fossils of this same genus are also to be met 

 with in the conglomerate at the base of Division l.c, either buried in its sandy calcareous 

 paste or wedged between the small fragments of slate, which in company with phos- 

 phatic nodules form the coarser part of this rock. 



It is true that the shells of Diplotheca are found, and indeed more plentifully in the 

 overlying shales, where they occur in company with those of Camerotheca ; but in these 

 upper measures they do not present us with the same thick, horny shells, which characterize 

 the individuals imbedded in the sandstones and conglomerates below. In other respects, 

 however (except in the apical appendage of one species), they do not appear to differ in 

 organization from the shells of this genus \vhi<h are found in the coarser beds in the lower 

 part of Division 1. 



From the positions in which the shells of Diplotheca, as above described, are Ibund, 

 we cannot but believe that the animals which inhabited them were shore-dwellers, and 

 therefore far removed in organization from the modern Pteropods. We find these described 

 by various authors as inhabitants of the open ocean, and as approaching the shores only 

 when driven thither by storms or currents. Another important feature in this class of 

 animals is the fin-like organs of locomotion from which they take their name, and the 

 absence of organs for climbing and to a great extent for prehension. We can hardly con- 

 ceive of the Diplothecae — living, as they did, in the sands of the sea-shore, or in the shallow, 

 sheltered bays,^ — as being in any respect similar in their mode of life to the Pteropods : they 

 appear to haA'e occupied a place in the economy of Nature which was subsequently taken 

 by the Grasteropods and Cephalopods, and, perhaps, the Lamellibranchs, and there is every 

 probability that in many respects they were constituted like the two former classes, to both 

 of which they present strong points of analogy. It is convenient, however, to retain them 

 under the head of Pteropods, to which in their general form and structure they appear to 

 have near affinities. 



