METAZOA VERMES. 267 



they attain this form they are said to be in the Obolella 

 stage of development. 



Each valve of Obolella had a rudimentary cardinal or 

 hinge area in which a groove was scooped out for the pedi- 

 cle. This groove is seen in an interior view of the bra- 

 chial valve (fig. 2) and still more clearly in the pedicle 

 valve (fig. 3). The markings in the interior of the valves 

 (figs. 2, 3) are the scars left by the muscles. 



One of the more progressive species closely related to 

 Paterina and Obolella, is Trimerella (PI. 651, fig. i, T. 

 ohioensis Meek, external view of pedicle valve ; fig. 2, inter- 

 nal view) which has a large hinge area, finely seen in fig. 

 2, and a furrow bounded by ridges, the inner edges of 

 which serve as teeth. This primitive mechanism is sug- 

 gestive of the more perfect articulation by teeth and 

 sockets of the more specialized Brachiopoda. 



Paterina, Obolella, and Trimerella represent the Atre- 

 mata, to which group of Brachiopoda Lingulepis (No. 652) 

 and Lingula (No. 654) also belong. In Lingulepis (No. 

 652, L. pinniformis Owen, = Lingula antiqua Hall) the 

 shape is rounded and similar to Obolella. The valves 

 are also unequal, and in other structural characters this 

 genus is intermediate between the Obolellidae and the 

 more specialized Lingulidae represented by Lingula. 



The power of resisting adverse conditions is so great in 

 Lingula that it has come down to us essentially unchanged 

 in structure from the early geologic period known as the 

 Ordovician, which followed the Cambrian. Its persist- 

 ence in time, according to Hall, 1 is "unequaled by that 

 of any other known genus of organisms." 



The embryonic shell of the Lingula living to-day is 

 similar to that of Paterina. PI. 653, figs. 1-3, represents 

 a closely allied genus, Glottidia albida Hinds. Fig. i 

 shows the Paterina-like shell (left unshaded in the draw- 

 ing) and the long hinge line ; also the youthful (nepkmic) 



1 Pal. N. Y., VIII, part i, 1892, p. 7. 



