Cumings — Morphogenesis of Platystrophia. 3 



having an index* of 1*5, and the greatest breadth about one- 

 third of the way from the beaks to the front margin. The 

 posterior margin (cardinal line) is straight, and the anterior 

 semielliptical. The greatest height is at the beaks, which pro- 

 ject slightly beyond the area, but are not incurved. The area 

 is considerably less than the width of the shell at the hinge ; 

 and the large foramen is about equally shared by the two 

 valves. At the apex of the ventral foramen is a small callosity. 

 No deltidium was observed. 



Ventral valve. — In the initial specimens, this valve, for 

 about half the distance from the beak to the front margin, is 

 without longitudinal markings of any sort ; but shows a num- 

 ber of very faint concentric lines. The anterior half of the 



Fig. 1. a, Nepionic stage of Platystrophia, from specimen obtained at 

 Vevay, Indiana, a, dorsal valve, showing median sinns and four plications 

 on either side, profile to left ; a', ventral valve showing median fold and 

 four plications on either side, profile to right; x 22. 6, c, Orthis lenticularis 

 var. lyncioides Matthew ; figures of the types ; b, ventral valve of a trans- 

 verse individual showing bifurcation of several plications toward the ante- 

 rior margin ; c, ventral valve of a narrower individual with very strong 

 plications. The umbonal region is exfoliated, a, Author's collection. 



valve is thrown into nine low rounded plications. One of these 

 occupies the median line, originates nearer the apex, and is 

 slightly more prominent than the others, thus forming a 

 median fold. Toward the front margin the plications are very 

 faintly crenulated by obscure concentric lines. 



"22. Terebratulites biforatus 



" Ein freyes ganz vollstandig, mit versteinerter Schaale erhaltenes Exem- 

 plar, aus dem sudlichen Frankreich. Vielleicht gleichfalls aus Kreide-lagern 

 der Champagne. (1 ex.) 



"Mit ganz gleichen, breiten, langlichrunden Halften, deren Schnabel 

 gleichformig gewolbt und auf beyden Seiten durchbohrt sind. Die Ober- 

 schaale mit einer breiten concaven Riickenfursche, die untere Halfte mit con- 

 vex hervorstehenden Riieken. Beyde Halften gleichformig der lange nach 

 gestreift, mit ziemlich tiefen zwischenfurschen. Ausserordentlich seiten." 



Die Petrefactenkunde auf ihrem jetzigen Standpunkte, etc., Gotha, 1820, 

 p. 265. 



No figure is given. 



* Breadth divided by length = shell index. 



