38 Cumings — Morphogenesis of Platystrophia. 



ditions and this may account for the sudden decline and 

 extinction of acutiUrata. 



As to the taxonomic value of the term acutiUrata the writer 

 is of the opinion that the form should certainly be considered 

 as a species, although, as pointed out, it is perfectly connected 

 by intermediate forms with P. laticosta. It is not, however, 

 except in a very limited zone associated with the latter variety ; 

 and it represents such a remarkable distinctness and complete- 

 ness of history after its initiation, that no one need confuse it 

 with any other member of the genus.* 



Platystrophia costata.f — It will not be necessary to enter 

 here into the question of the synonomy of this type. DeYer- 

 neuiPs objection to Pander's name was based upon the pre- 

 occupation of the term costata by Spirifer costatus Sower by. 

 Since the latter is a true Spirifer we must return to Pander's 

 name for the present form. 



Platystrophia costata (~P- dentata Meek ; P. crassa James) 

 makes its appearance in American faunas in the lower Lorraine 

 of the Ohio Valley ; and is the well known variety with a 

 rather small, gibbous shell having one plication in the sinus 

 and iive or six on the lateral slopes. The derivation of this 

 type has been discussed at sufficient length under the head of 

 neanic stages of Platystrophia. It comes from a normal tri- 

 plicate type by the dropping out of both secondary plications 

 of the fold and sinus. 



Under P. laticosta, mention was made of a uniplicate form 

 of that variety. Doubtless the latter has usually been confused 



* It may seem to be doing violence to taxology to distinguish a form at one 

 stage of its history as a variety and at another stage as a species under a dis- 

 tinct name ; nevertheless, I believe we must be prepared to take this step, 

 since we must certainly find as the investigation of fossils becomes more 

 minute and precise that cases such as this of Platystrophia laticosta and 

 P. acutiUrata are by no means exceptional. In this connection I may quote 

 a passage from a timely article by Mr. O. F. Cook on categories of species 

 (Am. Nat., vol. xxxiii, 1899, p. 292). He is comparing existing species to 

 islands and bodies of land more or less separated from each other in ' ' the sea 

 of non-existence." He says, speaking of incipient species, " Although the 

 designation by name of the various prominences or arms of a diversified 

 island [which is gradually sinking] is desirable, even before the expected 

 separation occurs, the prophetic tendency should, in the interest of historical 

 accuracy, be curbed to the extent of distinguishing in category between groups 



which are already segregated in nature and those which are not, by 



treating them as already distinct we ignore the existence of intermediate 

 forms and proceed as though degree of apparent difference were an index of 

 segregation or a taxonomic substitute for it." He suggests the use of the 

 term subspecies for all such non-segregated groups or incipient species. In 

 this sense P. laticosta would be a subspecies of P. lynx; and P. ' acutiUrata 

 the completely segregated group or species. I have retained the more usual 

 designation of variety for the former. 



f Pander, Beitrage zur Geognosie des russischen Reiches, 1830. p. 96, 

 pi. 11, fig. 3. — De Verneuil, Geol. de la Russie, 1845, p. 140. — Sowerby, Trans. 

 Geol. Soc. Lond., 2d ser., vol. v, pi. 55, figs. 5-7. 



