i 3 o SCIENCE PROGRESS 



forms a complete protection to the animal, shows several points 

 of resemblance with that of the Palaeozoic Bellerophontidce, not 

 only in the general shape but also in possessing a well-marked 

 keel as well as the characteristic notch {Atlanta, fig. 21); in 

 the closely allied Oxygurus (fig. 22), as well as in the other 

 shelled Heteropoda (in which the shell is only large enough 

 to protect the visceral hump — e.g. in Carinaria and Cardiopoda), 

 the notch is no longer present, although the keel is still 

 prominent in Carinaria (fig. 23). In the Pterotracheacea (Ptero- 

 trachea, Firuloides) the shell is absent altogether, the visceral 

 hump is reduced and the body has undergone still further 

 elongation. Indeed, the group of the Heteropoda or Natantia 

 shows its successful character, not only in a considerable degree 

 of differentiation in their organs (e.g. elaborate tooth-ribbon, 

 highly organised eyes, fin-like development of the mesopodium 

 and tail and separation of the sexes) but by their abundance 

 in numbers; for the shells of Atlanta pcronii (fig. 21) and 

 Oxygurus Keraudrenii (fig. 22) are sometimes found in very 

 considerable quantities in the Globigerina-oozz. 1 There is also 

 a tendency towards increase in size, for a Carinaria over two feet 

 in length was captured in 1898 during the voyage of the Valdivia. 



In the Pelecypoda it is obvious that since their bivalve shells 

 act (apart from their protective role) essentially as plates of 

 attachment for the powerful adductor muscles, it is impossible 

 for the enclosed animal to dispense with them. Hence it is not 

 surprising that, when the members of a family like the Hippuri- 

 tidce became so extraordinarily massive as to allow very little 

 room for the organism and became grouped into masses rival- 

 ling coral-reefs, they died out completely under altered con- 

 ditions, since they were utterly unable to dispense with their 

 shells or even to reduce their heavy armour, as in the case 

 of the different groups of Gastropoda which have just been 

 discussed. The theory of Steinmann, 2 that no group ever 

 becomes extinct and that the descendants of the Hippiiritidce 

 are to be found in the Tunicata, is so violently opposed to our 

 knowledge of the anatomy and vertebrate relationship of the 

 latter group, that it may be merely mentioned to be dismissed. 



In another class of animals— the Crinoidea — in which cal- 

 careous armour plays a still more pronounced role than in the 



1 Thomson, Sir Wyville, Voyage of the " Challenger? i. p. 121. 



2 Die geologischen Grundlagen der Abstammnngslehre, Leipzig, 1908. 



