470 Proceedings of the Royal Physical Society. 



glabra, on the other hand, persisted from the Devonian on 

 into the Carboniferous. Indeed, throughout the Brachiopoda 

 any sudden development of some extraordinary character or 

 large growth in size seems to have been followed by as 

 speedy an extinction. In the Mesozoic there is a great 

 falling off in the number of genera and species, and the 

 forms are chiefly restricted to the persistent Terebratula and 

 Rhynchonella types. Amongst these there is much slight 

 variation in the external markings, accompanied by great 

 persistence of type, but we also get outstanding specialised 

 forms in Lyra, Magas, Kingena, and Trigonosemus, and 

 these are strictly Cretaceous, whilst Pygope and Dictyothyris, 

 other specialised branches of the Terebratula stock, have 

 limited ranges in the Jurassic. Throughout the whole of the 

 time from the Lower Palaeozoic onwards, the Lingula, Crania, 

 and Discina types have persisted and are still existing. 



These three types of the Inarticulate division of the 

 Brachiopoda have shown very small range in variation 

 throughout, and their appearance has been very stable, but 

 never in great abundance. They may be truly termed per- 

 sistent types. It is in this division of the Brachiopoda that 

 hermaphroditism occurs, so rare a condition in the class. It 

 is noteworthy that the existing Lingula shows great resist- 

 ance to death, and seems to have the power of surviving after 

 being out of water and in a dry condition for some time. 



Turning to the Lamellibranchs, we find very persistent 

 types with exceedingly small amount of variation in 

 Solenomya, which has persisted since Carboniferous times 

 with very little change, and Nucula, which ranges from the 

 Silurian onwards. Curiously enough, both these forms are 

 classified by zoologists in the primitive group Protobranchia, 

 with exceedingly simple gills and a sole for creeping on the 

 foot. As a contrast to these, and showing great specialisa- 

 tion, and often to an extraordinary degree, and all with 

 limited range in time, we may compare the forms grouped 

 together as Eudistse. Such are Diceras from the Upper 

 Jurassic, and Bequienia, Monopleura, Caprina, Spherulites, 

 and Hippurites, and several other less known forms from the 

 Cretaceous. These extraordinary molluscs had a world-wide 



