Cuming s — Morphogenesis of Platystrophia. 123 



From tlie hiforata stock were derived the Ordovician types 

 dentata and fissicostata. Both hiforata and dentata passed on 

 into the Silurian, the former becoming extinct in the Clinton 

 (in this country at least), and the latter in the Niagara, after 

 assuming markedly primitive characters.* 



The facts presented above are expressed diagrammatically on 

 figs. 26 and 27. Fig. 26 shows anterior views of the forms of 

 Platystrophia, characterizing the several geological divisions 

 from the Cambrian to the Lower Devonian. One striking fact 

 brought out by the diagram is the progressive increase in size 

 (bulk) of the forms up to the culmination (acme) in the upper 

 Lorraine ; and the decrease in size from that point on. Another 

 feature is the rapid multiplication of new forms in the Lorraine, 

 and especially in the lower Lorraine. Another characteristic, 

 which this diagram is especially designed to show, is the two 

 types of sinus during the Ordovician : the angular ( VI, VII, 

 VIII, XI, XII XIII), and the rounded (777, IX, X, XIV). 

 These two types of sinus correspond to the two lines of descent, 

 namely, from a pauciplicate and a multiplicate stock, respec- 

 tively, as is shown in fig. 27. The Silurian forms find their 

 nearest American Ordovician parallels, so far as this feature is 

 concerned, in the lynx group. The European Ordovician 

 forms, except dentata, are not shown on either fig. 26 or 27. 



Fig. 27, in which the Roman numeralsf correspond to those 

 of fig. 26, is intended to express the ontogeny of each form as 

 well as the phylogeny of the entire group. This figure shows 

 several adult forms not shown in fig. 26. These are Platy- 

 strophia lynx of the upper Richmond (XVIII) ; P. dentata 

 of the Magara of the Mississippi Yalley (XIX) ; P. latioosta 

 of the lower Richmond (XI a ) ; gerontic P. eostata of the 

 upper Lorraine (YI a) ; and JBilohites verneuiliana of the 

 Clinton (Gotland). In this figure the forms below the hori- 

 zontal lines, to which Arabic numerals are affixed, are onto- 

 genetic stages of the forms above the horizontal lines, to which 

 Roman numerals are affixed. It will be noted that the nepionic 

 stage is the same for all except Bilobites (cf. 1-7 and 8-9). 

 The neanic stage is the same for all the Ordovician forms 

 (10-14), and has invariably one plication in the median line of 



* There are several specific and varietal designations that have been 

 applied to various forms of Platystrophia, which have not been mentioned 

 above. These are P. dorsata Hisinger sp. (Leth. Suec, 1837, p. 76) ; and 

 P. tridens and P. terebratuliformis McCoy sp. (Syn. Sil. Foss., Ireland, 

 1846, pp. 37, 38). Of P. dorsata deVerneuil says : " The Atrypa dorsata 

 which we place in the synonomy of this species [P. hiforata'] has the plica- 

 tions more numerous, and might be considered as a variety of it." I have 

 seen no specimens of this form, but probably deVerneuil's disposition of it 

 is correct. P. tridens McCoy, as shown by his figures, is a typical P. den- 

 tata ; and P. terebratuliformis is evidently a P. fissicostata, i. e., it belongs, 

 to the hiforata group. 



f Except IV, which is a pauciplicate P. lynx, the homceomorph of P. dentata^ 



