BRACHIOPODA. 



145 



angle with the lateral branches of the loop, and is deflected posteriorly ; its 

 surface is generally cylindrical, but in Nudeospira concentrica it is considerably 

 flattened. 



Fig. iw. 



Fig. 129. 



Fig. ISO. Fig. 131. 



Fiff. 12*f. A iireparation of Nudeospira ventricosa. Hall; showing the umbonal blades, the loop and the form of the 

 first volution of the spirals. 



Figs. 129, 130. The primary lamell.-e and loop of Nudeospira i-eniricosa. Hall. 



Fig. 131. A preparation of A'ucZeos^fra ct»?icinrta, Hall; showing one-hall of the hi-achitlium, the mode of attach- 

 ment of the crura to the umbonal blades and the flattened stem of the loop- (0.) 



The structure of the hinge-plate and brachial apparatus is now known in 

 the following species of this genus : N. pisuni, Sowerby, of the Wenlock lime- 

 stone; N. pisiformis, Hall, of the Clinton and Niagara group; N. ventricosa. 

 Hall ; N. elegans, Hall, of the Lower Helderberg, and N. concinna, Hall, of the 

 Corniferous and Hamilton groups. Besides these, the following American 

 species have been described: N. concentrica and N. rotundata, Whitfield, of the 

 Lower Helderberg group, and N. Barrisi, White, of the Kinderhook. 



A single individual of what appeared, from external characters, to be the 

 species Nudeospira concinna. Hall, from the Corniferous limestone of the Falls of 

 the Ohio, affords a surprising variation in the structure of the loop. The 

 internal parts have the same development and mutual relations as in specimens 

 of the species from the Hamilton shales, except that the stem makes a slightly 



