Cuming s — Morphogenesis of Platystrophia. 31 



some idea of the variation of laticosta / e is a normal individual 

 with one large (median) and two small (lateral) plications in the 

 sinus, a has but one plication in the sinus and the fold is 

 extremely elevated, h to d are intermediate between these 

 two. In seeking the derivation of this type as in other types 

 of Platystrophia the growth stages are of paramount import- 

 ance. In fig. 15, c, are shown two lateral plications in the sinus 

 originating at the usual distance from the beak but disappear- 

 ing a little over half way from the beak to the front margin. 

 Fig. 16, A, shows the inception of this process of reduction of 

 the lateral plications of the sinus. Here only one plication is 

 affected, and that only near the front margin. Fig. 16, B y 

 shows the same tendency in P. costata. 



Considering now the growth varices (fig. 16) it is evident 

 that laticosta becomes progressively more transverse during 

 ephebic stages, while costata becomes progressively less trans- 

 verse ; the early stages of the two being identical and also 

 identical with the early stages of P. lynx. In P. laticosta the 

 cardinal angle is progressively 72°, 73°, 63°, 68°, and 70°, 

 returning thus in gerontic stages to near the angle of an early 

 ephebic stage. In costata. (fig. 16. B) the angle changes from 

 95° to 78° to 82° to 99° in the fully adult stage,^ while in gerontic 

 stages it may be as much as 113°. The largest angle seen in 

 any stage of laticosta was 95°. Both laticosta and costata are 

 therefore derived from a primitive lynx by a reduction of the 

 number of plications, and an elevation of the fold. The rela- 

 tion of laticosta to costata is not a linear one ; but after the 

 establishment of a pauciplicate stock like the young of both 

 (see ante), a divergence occurred, one branch taking the direction 

 of an elongate narrow type (costata) and the other the direction 

 of a transverse acuminate type with extremely high fold (lati- 

 costa). 



The above mentioned uniplicate laticosta (fig. 15, a) is 

 found in the Upper Lorraine and is not to be confused with 

 P. costata. The former has an exceptionally high fold but the 

 same contour as a normal laticosta, from which variety it is 

 derived, and with which it is connected by every possible 

 gradation. If it is desired to distinguish this form from P. 

 laticosta, it may conveniently receive the name of unicostata* 



In the Dalmanella Meeki zone which intervenes between 

 the Lorraine and Richmond beds, no specimens of Platystro- 

 phia occur, except at the very top of the zone where the typical 



* The dentata and crassa of American authors undoubtedly include these two 

 forms costata and unicostata. The former term will be shown to apply only 

 to certain foreign and Upper Silurian biplicate types. Since the term crassa 

 confuses two distinct forms it had better be abandoned altogether, especially 

 since the form to which it was intended to apply is the costata of Pander. 



