6 PALAEONTOLOGY OF NEW YORK. 



angles, which develop from the sides of the delthyrium and meeting, enclose 

 wholly or partially a circular or oval pedicle foramen. At normal maturity 

 these plates become anchylosed along the median suture and form a single con- 

 vex plate (the so-called psmdodeltidium). 



The usual absence of the deltidium may be due either to accidental removal 

 or to resorption with advancing growth. In the adult and senile stages of de- 

 velopment many species, especially in the line of development to Sybingothyris, 

 form a testaceous callosity in the pedicle-cavity, thickening the umbo and ex- 

 tending across the delthyrium, reaching in extreme cases, nearly to the cardinal 

 margin. 



The muscular area consists of a subtriangular pedicle-impression occupying 

 the pedicle-cavity, and continuous with a deeply impressed oval or obcordate 

 area, which is posteriorly situated and divisible into a narrow median adductor 

 and broad lateral diductors, the surface of the latter being marked by radiating 

 or racemose furrows. The posterior and anterior members of the diductors 

 may frequently be distinguished, the former being of less extent and their sur- 

 face markings somewhat different from those of the latter. 



A median septum in this valve is usually absent ; occasionally it is in a con- 

 dition of incipient development, and in certain species having the aspect of 

 Spiriperina and belonging to the line of descent of which this genus may be 

 regarded as the final or accessory product, it forms a most conspicuous feature 

 of the interior. 



In the brachial valve the umbo is inconspicuous, the apex only being in- 

 curved over the cardinal area ; a median fold corresponds to the sinus of the 

 opposite valve. The cardinal area is narrow and divided by a broadly triangu- 

 lar delthyrium. The dental sockets are narrow, moderately deep and bounded 

 interiorly by highly developed socket walls, the extremities of which support 

 the crural bases. 



The cardinal process is a low, transverse, sessile apophysis, having its surface 

 vertically striated ; occasionally it is bipartite or it may be wholly resorbed. 



The crura are long, straight and slightly divergent ; their union with the 

 primary lamella of the spiral ribbon is at a broadly obtuse angle. The brachial 



