120 PALEONTOLOGY 



volute shell, distinguished from Cypvaa, etc., by the shape 

 of the aperture (narrow posteriorly, widened anteriorly). 

 There is also a pelagic group known as the Pteropoda, 

 occupying a similar position among opisthobranchs to 

 that of the Heteropoda among Streptoneura, their shells 

 having become superficially symmetrical, and very thin 

 and translucent, by adaptation to a swimming life in the 

 open sea. 



The Pteropoda, according to the investigations of 

 Prof. Pelseneer of Ghent, do not form a natural group, 

 being composed of two series derived from separate 

 families of opisthobranchs. Nevertheless the name is a 

 convenient one and may well be used if it is understood 

 that it has not an exact taxonomic value. Pteropods 

 abound in all the modern oceans to such an extent that 

 where there is moderately shallow water far from land 

 their shells form an important proportion of the forami- 

 niferal ooze on the sea-floor so-called pteropod ooze (at 

 greater depths their thin shells dissolve before reaching 

 the bottom). In the Cainozoic rocks of the Mediter- 

 ranean region, similar pteropod-bearing deposits are 

 known, and some pteropod shells have been described 

 from the Cretaceous ; but the entire absence of ptero- 

 pods from the pelagic Chalk of Europe, where they 

 might particularly be expected, seems to show that this 

 specialized group was then at the very beginning of its 

 existence ; and it is not surprising that the Jurassic 

 and Triassic rocks contain no certain remains of ptero- 

 pods. It is therefore startling to find in the Devonian 

 rocks abundant shells indistinguishable from those of the 

 modern pteropod Styliola ; and from thence downwards 

 to the Cambrian there are found a number of genera 

 which, though less closely similar to modern forms, are 

 yet more like pteropods than anything else (Hyolithes, 

 Fig. 35,^, Conularia, Tentacnlites, etc.). Recently, among the 



