44 CARBONIFEROUS AMMONOIDS OF AMERICA. 



parallel and the breadth increases very slowly; the abdominal shoulders 

 are nearly square, and the abdomen flat. The whorls are deeply embrac- 

 ing and increase rapidly in height. The umbilical shouders are square, the 

 umbilicus narrow and deep, and increases slowly in diameter. 



Dimensions. — The specimen, which was septate throughout, gave the 

 following dimensions: 



Millimeters. 



Diameter 34. 



Height of last whorl from mnbilical shoulders 17. 5 



Breadth 10. 



Width of umbilicus 7.0 



This gives the proportions: 1 : 0.5 : 0.29 : 0.20 : which agree almost 

 exactly with Karpinsky's figures, 1 : 0.5 : 0.30 : 0.20. On the Arkansas 

 specimen the involution is shown by the height of the last whorl from the 

 top of the next inner one, 12.5 mm. as compared with the total height of 

 the whorl, which is 17.5 mm. No measurements of this relation were shown 

 on the Russian specimen. 



This description applies only to the adult shell, the relative measure- 

 ments of the nepionic and neanic shells being very different. The Arkansas 

 specimen showed only the last whorl, but the young stages have been 

 worked out by Karpinsky," from whose work the following descriptioil is 

 translated: 



Around the cylindrical embryonic chamber [PI. XIII, fig. 2] are coiled very 

 evolute whorls, whose involution increases gnidually, but at first only in slight 

 measure [PI. XIII, fig. 4]. So, for example, the fourth whorl embraces at the begin- 

 ning only about one-fourth of the preceding; thus the height of the evolute portion 

 of this fourth whorl is six or seven times as great as that of its own involute portion. 



With later stages of growth the involution increases so that the whorls become 

 finally completely embracing, and probably conceal a portion of the umbilicus. 

 Because of this mode of growth the umbilicus appears at first broad, and increasing 

 rapidly, then only gradually, and finally not at all, while the whorl continues to grow 

 in height with great rapidity. Thus, at a diameter of the whorl of 4 or 5 millimeters, 

 the umbilicus is about one-half of the total diameter, and at 30 millimeters only 

 about one-fifth. The first and second whorls have a broad elliptical cross section 

 [PI. XIII, fig. 3], while that of the succeeding whorls becomes higher, with the long 

 elliptical axis vertical [PI. Xlll, fig. 2], and then finally the flanks are bounded by 

 almost parallel lines and the siphonal side is only slightly arched. 



Ontogeny. — According to Karpinsky the first or typembryonic stage is 

 latisellate — that is, the suture consists of a broad abdominal saddle; this 

 saddle is next divided by a broad siphonal lobe (PL XII, fig. 1). 



a Ammoneen der Artinsk-Stuf e, p. 8. 



