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THE QUARTERLY REVIEW OF BIOLOGY 



be shown to a very slight degree in species 

 of Heteroceras or Nostoceras, but which in 

 its extreme development in Nipponites is, 

 for the present, highly anomalous. As in 

 the Gastropod genus Vermetus, Nipponites 

 was undoubtedly derived from a trochoid 

 coiled ancestor. 



THE PROBLEM OF SEPTA AND SUTURES 



The familiar septal and resulting sutural 

 progression from nautiloid (smooth 

 curves), through goniatitic (angular) and 

 ceratitic (frilled lobes), to ammonitic 

 (frilled or digitate lobes and saddles), is 

 well known, and in a general way is an 

 epitome of the evolution of these parts. 

 It is not, however, a simple linear series. 

 Early forms often show unusual specializa- 

 tion, and later forms simplification, but 

 it is well to avoid the philosophic impli- 

 cations of the terms acceleration and 

 retardation, so much a part of the vocabu- 

 lary of the cephalopod students of the 

 penultimate generation. 



A comparable change from septal sim- 

 plicity to septal complexity may be 

 observed in the ontogeny of the more 

 evolved types, and this is the basis for the 

 enormous literature on the biogenetic law 

 as exemplified by the shelled cephalopods. 

 It is commonly assumed that the sutural 

 are the most constant features, and that 

 however much the body-form or ornamen- 

 tation may vary, the court of last resort 

 in determining genetic relationships is 

 the suture, which is said to be constant in 

 species when individuals of the same de- 

 gree of maturity are compared. This has 

 now become a tradition among students of 

 ammonites, but like all similar traditions, 

 it is unsound, and the only way to es- 

 tablish it would be for systematists to 

 consider every slight variation indicative 

 of distinct species. In an account, shortly 

 to be published, of the ammonite faunas 



of Peru, a student of mine, M. M. Knechtel, 

 has shown considerable variation of single 

 sutures on the two sides of the shell, and 

 similar features have occasionally been 

 recorded by earlier workers. In such a 

 case the dogmatist must either admit 

 sutural variation or contend that the two 

 halves of the same shell belong to different 

 species. 



It is not my purpose in this essay to go 

 deeply into this vexed question, with all 

 its implications, but it may be stated as a 

 general principle that the taxonomic value 

 of such features as sutural pattern, shell 

 form, ornamentation, etc., will vary 

 greatly from genus to genus. In some 

 cases one and in other cases another feature 

 will be entitled to the greatest weight, 

 but there will be no single magical 

 criterion. 



The factors that led to the increasing 

 complexity of septa in the ammonoids 

 have been a favorite field for speculation 

 since von Buch first emphasized its 

 existence by proposing the so-called genera 

 Goniatites, Ceratites, and Ammonites. The 

 subject remains obscure to the present, 

 and none of the suggestions that I have 

 encountered seem satisfactory. That it 

 is related in some way to habit would seem 

 probable by the essentially goniatitic 

 form of the sutures in such a nautiloid as 

 Aturia, which every beginner thinks is an 

 ammonite; but that benthonic forms can 

 be distinguished from planktonic or nec- 

 tonic by the characters of the sutures, as 

 Schmidt implies in a recent paper, or that 

 Keferstein's wholly hypothetical "pre- 

 septal gas" shows that animals with 

 highly complicated sutures were divers, 

 is wholly without foundation. 



In a way progressive sutural complexity 

 is to be correlated with increase in bodily 

 size in excess of increase in shell capacity. 

 This is also a factor in the corresponding 

 sutural modification during ontogeny, for 





