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, “preference 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 Palzon- 
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 preservation to the spine figured by St. John 
and Worthen in Vol. II. of the Illinois Paleontology, 
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 
Fie. 12. of which show tuberculated cost#, and the absence 
Ctenacanthus gracil- of this character in Newberry’s spine is probably due 
limus N.& W. St. to abrasion. One example in particular, from the St. 
Louis Limestone; on sige : 
Missouri. Portion LOWS limestone, and belonging to the Museum of 
of ornamentation, Comparative Zodlogy, exhibits the finer ornamenta- 
x S: tion very distinctly in its proximal portion (ef. text- 
figure 12) where the tubercles are seen to be small, 
stellated, and rather widely spaced in proportion to the extreme fineness 
of the costz. 
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 OC. gractllimus 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.1 Examples denuded of their 
2 Ann. Mag. Nat. Hist. (2), Vol. II (1848), p. 117. 
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