NO. 2297. BRACHIOrOD GENUS PLATYSTROrillA—iloEWAN. 387 



specimens shovr that the sinus is proportionally deeper than the height 

 of the fold. This is accomplished by the compression of the pedicle 

 valve, which results in a greater elevation of the slopes next to the 

 sinus. 



With few exceptions the surface of both valves is marked by strong 

 plications, which are generally sharp. Platystrophiafernvalensis, new 

 species, from the Fern vale of Cape Girardeau, Missouri, is striated, 

 but next to the beak these striae merge mto broad plications. The 

 genus is extremely variable in form, size, and number of plications. 

 Form and size are reasonably constant, but the number of plications 

 varies considerably within the limits of the species. 



The surface layer of the shell is finely granulose. This feature is 

 not always evident if the surface l&jev has been subject to abrasion. 

 The inner surface is finely punctate. 



Old age is indicated by increase in gibbosity, thickening of the 



shell, strong growth varices, and loss of characters of low taxi- 



nomic rank. 



TYPE OF THE GENUS. 



King took as the type of his genus, Terehratulites hiforatus Schlot- 

 heim. This species was described in 1820 as follows: "Mit ganz 

 gleichen, breiten, langlichrunden Halften, deren Schnabel gleich- 

 formig gewolbt und auf bey den Seiten durchbohrt sind. Die Ober- 

 schaale mit einer breiten concaven Riickenfurche, die untere Halfte 

 mit convex hervorstehenden Riicken. Beyde Halften gleichformig 

 der Lange nach gestreift, mit ziemlich tiefen Zwischenfurchen. Aus- 

 serordentlich seiten." ^ 



Schlotheim's type material consisted of one specimen which was 

 not figured. His description is too general for application to any 

 particular species, and as a result paleontologists have not been able 

 to come to any definite conclusion as to the specific denomination of 

 the shell. 



M. de Verneuil says ' that Von Buch saw Schlotheim's type in the 

 Berlin Museum, and reported the distinctive characters to be the 

 presence of five ribs in the sums, and the width ]^roportionally greater 

 than the other species of the genus then known. There are individ- 

 uals of nearly every species and every variety of this genus which 

 have four, five, or more plications in the sinus, and transverse forms 

 are common. There are subquadrate specimens with five plications 

 in the smus and transverse specimens with five plications in the sinus, 

 so that the characteristics cited are not sufficient to constitute a 

 species. 



1 Schlotheim, Die PetrifactenkundeaufihremjetzigenStandpunktedurchdieBeschreibungversteinerter 

 Schalle, 1820. p. 265. 

 » De Vemeuil et De Keyserling, Geol. Russia, vol. 2, p. 138. 



