86 



bulletin: museum of comparative zoology. 



designated. It is a well-established principle of nomenclature that in 

 cases like this, or as between names the equal pertinency of which may- 

 be in question, " jiret'erence shall be given to that which is open to 

 least doubt " (A. O. U. Code, Canon XVII.). 



As a matter of fact, when these two "species" were united by 

 St. John and Worthen in 1883, some slight doubt was expressed as to 

 their identity, and the authors very properly chose for their common 

 designation that which was founded on the most perfect specimen, and 

 hence was open to least doubt, namely, C. gracillimus. By this decision 

 the spine figured in Plate XIII. Figure 3 of the Illinois Palseon- 

 tology. Vol. II., was definitely established as the type-specimen, and the 

 only question is whether it actually belongs to Ctenacanthus, or should 

 be removed to Acondylacanthus as was proposed by Newberry in 1889. 

 The latter author rests his claim upon a worn, im- 

 mature, and distorted specimen, now in the Museum 

 of Columbia University, and very much inferior in 

 point of presei'vation to the spine figured by St. John 

 and Worthen in Vol. II. of the Illinois Palaeontology, 

 to which no reference is made by Newberry. 



The correctness of St. John and Worthen's deter- 

 mination is confirmed by several additional specimens 

 which have come under the writer's observation, all 

 Fig. 12. ^f which show tuberculated cost£e, and the absence 



Ctenacanthus gracil- of this character in Newberry's spine is probably due 

 to abrasion. One example in particular, from the St. 

 Louis limestone, and belonging to the Museum of 

 Comparative Zoology, exhibits the finer ornamenta- 

 tion very distinctly in its proximal portion {cf. text- 

 figure 12) where the tubercles are seen to be small, 

 stellated, and rather widely spaced in proportion to the extreme fineness 

 of the costae. 



Just as C. xiphias (St. J. and W.), from the Keokuk limestone was first 

 assigned to Acondylacanthus on the evidence of a worn specimen, so the true 

 relations of C. gracillimus were rendered obscure by faulty preservation. In 

 the same connection it may be remarked that another very interesting group 

 of spines from the St. Louis limestone has been for the same reason misin- 

 terpreted by various authors. These are referable to the same species as 

 occurs in the Carboniferous Limestone of Armagh and Gloucestershire, and 

 described as Physonemus arcuatus by M'Coy.i Examples denuded of their 



1 Ann. Mag. Nat. Hist. (2), Vol. 11 (1848), p. 117. 



/m«sN. &W. St 

 Louis Limestone ; 

 Missouri. Portion 

 of ornamentation. 



xf. 



