CORBONIFFROUS CEPHALOPODS. 355 



mistakes between casts of Nautiloids and Ammonoids, if the sutures and 

 the young are entirely hidden. The species described below illustrates the 

 constancy of the sutures and the variability of the form. 



I wish also to call attention here to a characteristic not before noted in the 

 genus Paralegoceras. The second lateral lobes or divisions of the great 

 magnoscellarian saddles occupy the umbilical shoulders in Gastrioceras, 

 whereas in Paralegoceras these same lobes are situated on the sides of the 

 whorls. This distinction was noted in Gastrioceras in the description of this 

 genus,* but not in Paralegoceras. The latter also has shallower umbilici 

 and more rounded umbilical shoulders than in Gastrioceras, the third pair of 

 saddles appearing also partly on the sides. We are now able to cite another 

 species as belonging to this interesting genus. Paralegoceras (Gastrioceras) 

 Russiense of Marie Tzwetaev has a form very like Gast. compression as figured 

 by the authoress,* but the outlines of the sutures and umbilical shoulders 

 show that we are here dealing with a form of Paralegoceras. This is less 

 compressed than Parol. lowense, Meek and Worthen, and shows closer affinity 

 with Gastrioceras in the form of the whorl. 



Fig. 59. 

 Gastrioceras compressum, n. s. 



Loc. Colorado River. San Saba County, near Bend. Texas. 



Coll. National Museum. 



Fig. 57-59, natural size. 



The form of the whorl is helmet-shaped and at the diameter of 109 mm. in 

 a cast the greatest transverse diameter was 42 mm. ; the distance in a straight 

 line from umbilical shoulder to centre of abdomen 38-39 mm.; the abdomino- 

 dorsal diameter 23 mm. The increase by growth in both diameters is slow 

 and the umbilici consequently shallow. The involution covers more than two- 

 thirds of next internal whorl at the diameter of 109 mm. and in another 

 specimen at diameter of 68 mm. it is just two-thirds. The still younger 

 whorls are numerous and visible from the sides at the centres of the umbilici, 

 and doubtless the amount of involution is correspondingly less. Constrictions 

 appear in the smaller specimen measured and in the younger stages of another 

 flattened example. 



*Foss. Ceph., Proc. Boat. Nat. Hist, XXII, p. 32*7. 



*Ceph. du Calc. Carb. tie la Russie Centrale, PL VI, pp. 30-32. 



