Miss Agnes Crane — Evolution of the Brachiopoda. 107 



26), Waagen (68), Hall (38), Barranrle (1), etc., as considerably 

 extended and emended of late by Schuchert (64, 66). 



The Atremata, with a free pedicle-passage between both valves, 

 are the oldest and simplest forms of Brachiopoda, represented in the 

 Lower and Middle Cambrian by Faterina, Obolella, etc., but Lingida, 

 a surviving type, did not appear before the Lower Silurian period. 



Of the twenty-four known hingeless atrematous genera, no less 

 than nine appeared in the Primordial fauna. Of their immediate 

 ancestors, as yet we know nothing. Twenty-two in all appeared 

 during the Palaaozoio era. Two little " tongue-shaped " shells still 

 represent them — Lingula in the Eastern oceans and Glottidia in those 

 of the Western hemisphere. 



The Atremata gave rise almost simultaneously in the Primordial 

 seas to members of the orders Neotremata and Protremata ; for 

 types it is now generally conceded are more plastic at or near their 

 points of origin. 



The Neotremata comprises all the disc-like hingeless forms with 

 pedicle passage restricted to the ventral valve. These diverged from 

 the Atremata by way of the round punctured shells of the family 

 of the Trematidee. Thirty-one genera are referred to tliis order, 

 which is represented in existing oceans by the long-lived Crania, 

 Orbiculoidea — also known as Disr.ina, — and by the modern branchlet 

 of Discitiisca. 



The order Protremata includes the large number of 82 articulated 

 genera with a single plate deltidium, classed by Schuchert (64) 

 into two suboi'ders ' and fourteen families, many with but a short 

 range in time. The earliest forms of the primary division are con- 

 sidered to have originated from the Atremata, via the Cambrian 

 species allied to the genus Kutorgina {K. cingulata, Billings, type), and 

 were chiefly restricted to the Paleeozoic oceans. Others appeared 

 at the close of that great epoch, or with the dawn of Mesozoic time. 

 The only direct living representative of the Protremata, as 

 defined by Beecher, is the Tiiecidoid Lncazelln Mediterranea, Risso. 

 Other members of the family to which this species belongs are 

 known to occur in the Jurassic, possibly in the Trias. Then there 

 is a break in continuity of descent, but there is good reason to 

 believe that the remoter ancestry of the Thecidiidse will eventually 

 be traced through the adhering Productoids back to the Silurian 

 Strophomenoids, which have the same habit of attaching themselves 

 by one valve to foreign objects, or, like many neotrematous genera, 

 to each other. 



It was from the Protremata, prolific in short ranging genera, 

 that the important order of Telotremata originated in that family 

 of Pentameridte, abundantly represented in Silurian seas, and it is 

 believed by the genus Camarella in the Primordial oceans. It 

 diverged in two main lines of descent, with numerous collateral 



^ I. Trullacea, from trulla a scoop, for the spoon-shaped plate, the spondylium 

 of Hall. 

 II. Thecacea\ from theca a cover, referring to the deltidium in one plate 

 covering the three-cornered fissure in the apical portion of the 

 ventral valve. 



