648 MOLLUSCOIDEA. 



is what is known as the " deltidium." This is a triangular calcareous 

 plate, sometimes single, sometimes composed of two lateral halves 

 (fig. 492, d), which is developed in front of the foramen of the 

 ventral valve, and may either completely enclose the foramen (as in 

 Rhynchonella), or may simply form the lower boundary of this aper- 

 ture. What is called a " pseudo-deltidium," again, is a triangular 

 calcareous plate which grows downwards from the upper end of 

 the triangular foramen in such genera as Spirifera, and which 

 partially closes this opening. 



As regards their distribution in space, all the Brachiopoda are 

 marine, the number of known existing species and varieties being 

 under one hundred and fifty. The range of the living Brachiopods 

 in depth is very variable, even as regards individual species, some 

 forms commonly living between tide-marks, or in quite shallow 

 water, while others extend to depths of nearly three thousand fathoms. 

 Upon the whole, however, the Brachiopods must be regarded as 

 inhabitants of comparatively shallow water, since about half of the 

 known living species are confined to depths of less than one hundred 

 fathoms. 



Leaving the problematical Eozoon out of sight, the Brachiopods 

 are amongst the oldest forms of animal life, representatives of this 

 class appearing in the Lower Cambrian deposits. Owing to the 

 great number of fossil Brachiopods, and also owing to the fact that 

 particular types are commonly confined to particular geological 

 horizons, the study of this group of animals is one of great palse- 

 ontological and stratigraphical importance. Out of one hundred 

 and thirty-nine genera recorded by Davidson, one hundred and six 

 appeared first in the Palaeozoic rocks ; thirty-four genera are repre- 

 sented in the Mesozoic deposits ; and twenty-one in the Kainozoic 

 and Recent deposits taken together. The genera Lingula, Discina, 

 Crania, Terebratula, and Rhynchonella appear at some point or 

 another in the Palaeozoic series, and still exist. About eight genera 

 are represented in the Cambrian ; whereas no less than sixty-seven 

 genera are represented in the combined Ordovician and Silurian 

 deposits, after which the number of genera becomes progressively 

 reduced, fifty-two generic types being present in the Devonian, forty 

 in the Carboniferous, and twenty in the Permian. Of the known 

 Mesozoic genera about sixteen are peculiar to this period ; while of 

 the thirteen Tertiary genera no less than twelve survive at present ; 

 and there are eight exclusively recent genera. 



Upon the whole, the hingeless or Inarticulate group of the Brachi- 

 opods is the most ancient, all the Cambrian genera, with the excep- 

 tion of Orthis, belonging to this division. Of the Cambrian genera, 

 Lingula, Discina, and Crania still survive. While less than one 

 hundred and fifty species exist at present, the united Ordovician and 



