Cunnings — Morphogenesis of Platystrophia. 11 



of numerous individuals has, however, demonstrated the fact 

 that even this type is fundamentally triplicate.* The details of 

 this point will appear later. Suffice it to say that here again 

 the secondary plications have been suppressed. They may 

 often be seen more or less distinctly in the rostral region, 

 but fade out toward the front. With the one exception, 

 therefore, noted above, all the Ordovician forms of this country 

 belong fundamentally to the triplicate type. 



Comparison with adult stages of other Orthidce. — The early 

 neanic shell with one plication in the sinus and live on 

 each lateral slope is strikingly suggestive of the var. costata 

 of Russia and the lower part of the Cincinnati group of this 

 country. As just pointed out, however, the latter is evidently 

 derived from a triplicate type such as we find abundantly 

 represented in the Trenton. Practically any group of Platy- 

 strophia may produce an occasional uniplicate individual. Fig. 

 15/ shows it in the normal costata ; fig. 15« in the var. lati- 

 costa. In the lynx group it is extremely rare. Only one exam- 

 ple has come to my notice. Theoretically there should be a 

 uniplicate Platystrophia in the lowest Ordovician. If this 

 stage was passed before any definite differentiation of the fold 

 and sinus took place, then some of the small Orthidw of 

 the Calciferous and Chazy come very near the requirements. 

 Otherwise I know of no form corresponding to the theoretical 

 uniplicate type.f 



In some respects the adult Plectorthis plicatella resembles 

 the neanic Platystrophia. If the sinus and fold be disre- 

 garded (and in some Trenton forms these are surprisingly 

 inconspicuous), the neanic Platystrophia is almost a Plector- 

 this. There is little doubt that when the nepionic shell of 

 Plectorthis is discovered it will be found to be quite indistin- 

 guishable from the nepionic shell of Platystrophia, since the 

 two groups present at the beaks almost identical characters. 

 Plectorthis appears in American faunas in the Black River 

 formation,;}; and Platystrophia, as we have seen, in the basal 

 Trenton. Plectorthis may therefore represent an offshoot 

 from the Platystrophia group near its initiation or, as sug- 



* For the aniplicate European forms I cannot speak. From what I can infer 

 from a careful study of their range and variation, they also probably sprang 

 from a triplicate or biplicate type. This subject will be discussed more in 

 detail later. 



f I have gone over the large collections in the Yale Museum, from Crown 

 Point and other well known localities of Chazy and Calciferous fossils. No 

 doubt the connecting form between Orthis lenticularis and PlatystrophiawiM 

 come to light when much more complete investigations of these old Ordovi- 

 cian rocks are made. 



X Mr. P. E. Eaymond figures and describes specimens of Plectorthis plica- 

 tella from the Black River limestone of Crown Point, N. Y. (Bull. Am. Pale- 

 ontology, No. 14, 1902, p. 38, pi. 19, figs. 5, 6.) 



