Cunnings — Morphogenesis of Platystrophia. 23 



the section at Montreal, given in the Geology of Canada, " Orthis 

 lynx " is listed from the first, second, and fourth divisions of the 

 section ; that is from the lower 150 ft. of the formation. The 

 chances are that the present specimens are from the lowest of 

 these divisions, or less than 10 ft. above the Black River forma- 

 tion.* These small shells may therefore represent the immedi- 

 ate ancestors of the normal Trenton form above described. 



The small pauciplicate P. lynx 9 Us 



(fig. 9 bis) from the lower Lor- 

 raine at Cincinnati, is of interest 

 as the immediate progenitor of 

 P. costata and probably also of 

 P. laticosta. It occurs, asso- 

 ciated with the uniplicate form 

 (costata), often in clusters, one 

 of which is shown in fig. 9 bis. 

 This specimen has the shells in 

 their natural position, with the 

 beaks very closely ap pressed to 



the snrfaee of ^rmort The Fig. 9 6is. Cluster of Platystro- 

 tne sunace oi suppoit. ine pMa lynx ^ pauciplicate forni) on a 



individual at the top 01 the ng- colony of Constellaria constellata. 

 ure is wedged in SO closely All the individuals are in their nat- 



between the adjacent branches ural P^jtion with the beaks closely 



j. , ~ . 77 - ■ • t appressed to the surface of support. 



Ot the Constellaria, as tO llldl- a an d c, Plectorthis plicatella; b, 



cate that the two valves never Zygospira modesta. Collection of 



gaped more than very slightly tbe Museum of Comparative Zool- 



P l -. -, ■ ■ n j ogj, Harvard University. 



The question arises whether this form of P. lynx is not an 

 unmodified descendant of the Montreal type just described. 

 This is certainly possible if not probable. The occurrence of 

 an occasional individual with few plications among the normal 

 P. lynx of the middle and upper Trenton suggests, on the 

 other hand, that these lower Lorraine forms may have origi- 

 nated as a variant of the latter. For convenience this form of 

 P. lynx may very well be called pauciplicata. It seems, as 

 stated above, to represent the initiation of a tendency that after- 

 wards in P. laticosta and P. costata assumed considerable 

 importance. 



In American literature the Trenton shells have often been 

 referred to the species biforata. I have already pointed out 

 the fact that this type is not represented in American Ordo- 

 vician deposits above the basal Trenton. The large form from 

 the Cincinnati group universally known as Platystrophia 

 lynx probably differs from the Russian lynx and is identical 

 with our Trenton forms. In the interval between the Trenton 

 and Lorraine the typical lynx changed surprisingly little. Fig. 

 *Geol. of Canada, 1863, pp. 137-138. 



