MANUAL OF THE MOLLUSCA. 



couceiitrically striated. Dorsal valve with a longitudiDal 

 Beptum ; upper part of the ventral valve with a deep sub- 

 triangular muscular impression which unites with the rostral 

 cavity. 



The species of this genus are Meristce without the peculiar 

 appendage of the ventral valve. 



Distribution, 17 species. Silurian — ^Devonian. Europe, North 

 America. 



The forms marked by plications on the mesial fold and sinus, 

 and sometimes with obscure or distinct plications on the lateral 

 portions of the shell, constitute the genus LEiORHTlsrcHUS, Hall, 

 ■i species. Devonian. United States. 



Chabionella, Billings, 1861. 



Synonym, Cryptonella, Hall, 1861. 



Type, Athyris scitula. 



Shell resembling Athyris, but more elongate-ovate or ap- 

 proaching to Terehrattda in form. Internal spires as in Athyris 

 and Merista, but the dorsal hinge-plate is either obsolete along 

 the middle, or anchylosed to the bottom of the valve. Foramen 

 terminal, bounded on the lower side by one or two deltidial 

 pieces, or by a portion of the shell. The mesial septum in the 

 dorsal valve is either absent or rudimentary. 



Distribution, 15 species. Devonian. America, Spain. 



Ntjcleospira, Hall, 1859. 



Etymology, nucleus, and spira. 



Types, Spirifer pisum, Sowerby ; Nucleospira ventricus 

 Hall, Figs. 21, 22, 23. 



4 



Fig. 21. 



Fig. 22. 

 Nucleospira ventricosa. 



Fig. 23. 



Fig. 21 , interior of the dorsal valve. Fig. 22, interior of the ventral valve. Fig. 23, 

 interior of the dorsal valve, with a poriion of the ventral valve attached. 



/, cardinal process ; c c, crural processes ; b b, dental pockets ; r, muscular unprns- 

 Rions ; s, medio-longiludinal septum; tt, teeth; 2- a flattened space or false affea 

 beneath the beak. \HalL.) 



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