88 PALEONTOLOGY OF NEW YORK. 



o-rowth, developed earlier in some species than in others, and hence the term 

 was essentially a misnomer. On this account some authors, particularly the 

 French and German writers, have preferred to use the term Spirigera, pro- 

 posed by d'Orbignv in 1847* to replace Athyris, and founded on the same 

 species. The term Euthvris, also, was proposed by the late Professor Quen- 

 STEDTjf but it has not come into general use. Among generic appellations 

 there are too many misnomers which have an established and positive value, 

 to permit the rejection of the term Athyris without great inconvenience, 

 attended by no equivalent advantage. The term is therefore used in a re- 

 stricted application and substantial reasons will be given for a subdivision of 

 the genus.| 



The essential feature which forms the basis of union of all the variations of 

 the genus here discussed, is the nature of the loop. This complicated struc- 

 ture was first demonstrated by Davidson in 1857 § for the species Athyris 

 pecti?iifera, Sowerby. Since that date our knowledge of this organ has become 

 more extended and more exact, and we now know its peculiarities in several 

 species from the Devonian, Carboniferous and Permian faunas. 



Athyris concentrica, representing a combination of characters which expresses 

 the typical phase of athyroid structure, is distinguished from the subordinate 

 divisions of the genus by the following differences : 



(a) The usually transverse form ; this is a feature subject to variation, but 

 throughout the group this outline is striking, simply from the frequency 

 of its occurrence. 

 (h) The lamellar expansions or varices at the concentric growth-lines are 

 simple, that is, are not split up into spinules, nor do they embrace 

 such spinules, but are usually transversely striated. They are often 

 highly developed toward the margins of the valves, but are generally 



* Comptes rendus, vol. xxv, p. 268. 



t Petrefactenkunde Deutschlands, p. 442. 1871. 



J Students who may wish to follow the vai'iation of opinion in regard to the value of the genus Atutris, 

 are referi-ed to a paper by the late Mr. Billings, Palaeontologist of the Geological Survey of Canada, " On 

 the Classification of the Subdivisions of McCoy's .Genus Athyris, as determined by the Laws of Zoological 

 Nomenclature ; " American .Tournal of Science, vol. xliv (1867), p. 48. See, also, vol. xxxiii of the same 

 Journal, pp. 127, et seq. 



§ British Permian Biachiopoda, p. 21. 



