BRACHIOPODA. 11 



though losing none of its external characters. We may cite the apparent out- 

 come of Spiriferina from the septate lamellose Spirifers ; and Syringothyris as 

 the final product of changes along the line of those plicate Spirifers with smooth 

 fold and sinus. Such changes, then, which we recognize as having induced 

 actual generic differences are confined to the interior and the intimate structure 

 of the valves ; external variations, as far as now known, when unaccompanied 

 by internal changes, must be regarded as of altogether subordinate value. 



These external differences, however, make an excellent basis for a grouping 

 of the members of this protean genus, and one not merely conventional and 

 arbitrary, since it serves to indicate, within the integrity of the genus, lines of 

 progress leading to resultants which are no longer congeneric. 



Several authors have proposed a classification of the Spirifers on the basis of 

 their external ornamentation, and earlier writers were especially fond of 

 attempts in this direction. Frequently the proposed arrangements have been 

 vitiated by the inclusion within this genus of distinctly heterogeneous species. 

 Thus with VON Buch's classification in 1836 and 1837,* and to some extent 

 with that given by Phillips in 1836.t The latter writer proposed six subdivi- 

 sions of his Spirifers: a. Cuspidate; h, Angustatcz ; c, Radiata; d, Glabrata ; 

 e, Terehratuliformes; f, Filosce,. The list of species referred to the last two divi- 

 sions contained no Spirifers, but Athyris (e) and Schizophoria, Orthothetes 

 and Chonetes (f). The Cuspidate were typified by S. cuspidatus, and this group 

 is equivalent, in a restricted sense, to the genus Syringothyris, though it was 

 also made to include species of Cyrtia, Derbya and Orthothetes. The Angus- 

 tat(B and Radiate were separated on the basis of the length of the hinge, which, 

 in the former, equaled or exceeded the width of the shell. Both included 

 species with smooth and plaited fold and sinus. The Glabrate included both 

 Spirifer glober and S lineatus, that is, both Martinia and Reticularia. 



De Verneuil J divided the Spirifers as follows: \. Anormaux, mc\\\(\.mg the 

 equirostres (== Porambonites) and bifores (= Platystrophia) ; II. Lisses, equiva- 

 lent to Reticularia and Martinia, but embracing a species of Tkiplecia ; III. 



* Ueber das Genus Delthyris ; and Uebei- Deltliyris, oder Spirifer iind Orthis. 



t Illustrations of the Geology of Yorkshire. 



I Geologie de la Russie de I'Europe, p. 127. 1845, 



