BRACHIOPODA. 7 



faunas appearing on the earth, its persistence in time is nevertheless unequaled 

 by that of any oilier known genus of organisms. 



The palaeozoic Lingulas, sensu strido, have not shown variations of sullicient 

 importance to permit a thoroughly satisfactory subdivision of the genus. As 

 the interiors and muscular scars of different species become known, they re- 

 veal a closer alliance with one another, and with the recent type of the genus, 

 than had been suspected. 



There is a growing tendency among authors to remove certain species from 

 this genus to Lingulella on account of an apparent inequality in the valves of 

 the shell, accompanied by a cardinal area on the pedicle-valve, which seems 

 to be grooved or depressed from the apex forward. Great care is needed in the 

 accurate determination of these features. The type species, Lingula anatina, 

 has not only gaping and slightly unequal valves, but the pedicle-valve has a 

 distinct cardinal shelf or area, divided longitudinally by a depression widening 

 from the apex anteriorly, and termed by Ktxg* the " deltidium." This, accord- 

 ing to Professor King, is usually not well developed, and in the average speci- 

 mens of L. anatina found in museum collections, ap- j<^ 

 pears to be partially or quite wanting, from accidental ^' "^^ ^^\. 

 causes. The deltidium is bounded at the sides by ^""j^^/ S^^^^v 

 elevated ridges, which, at the anterior ends, are each ^;^3^^^^^^^^^^^-^^ 

 developed into a small callosity. "The ridge-callosi- ^lo. 2. xi«<7«;aana«,m,-c.-.niinaiarea 



,. 1 1 .*••/* i. ii 1 xu of pedicle-vaUe, alter King, a, del- 



ties are no doubt nisignifacant; nevertheless they tidiuni;6,.ieitidiaiii.jgc8;c,cieuidiai 



,1,1 T , ,. • , J i. i callosities ; rf, areal borders. 



appear to be the rudmients 01 important structures. 



Apparently they have become so far developed in Lingula Lesmuri, as to serve 



to articulate the valves. If I am correct this species cannot 



belong to the genus in which it has been placed."! In the opposite or brachial 

 valve is a slight area without a deltidium, but bearing concentric growth-lines, 

 and a very faint longitudinal groove. 



It is to be expected that we shall find similar features more or less devel- 

 oped among the fossil Lingulas. The broad, depressed deltidium of L. anatina 



* Anuals and Magazine of Natural History, July, 1873. 

 t King, loe. cit., p. 13. 



