UPPER TRIASSIC SPECIES. 109 



Genus CLYDONITES, Hauer. 

 Olydonites l^vidorsatus, Hauer (sp.). 



Plate 10, fig. 7. 



Ammonites Icevidorsatus, Hauer (1860), Sitzungsb. K. Akad. Wien, pi. 3, figs. 9 and 10 b. 

 Goniatites Icevidorsatus, Gabb (1864), Geol. Survey of California (Palaeont.), I, 21, pi. 3, 

 figs. C, 7. 



Shell compressed-discoidal, witli a very wide, exceedingly shallow um- 

 bilicus; periphery more or less narrowly rounded; volutions numerous, very 

 slender, increasing very gradually in size, and each one enveloping about 

 one-thii'd to one-half of the next within ; aperture as determined by sections 

 of the volutions, emarginate-subelliptic, being more or less emarginated on 

 the inner side for the reception of the outer side of the next turn within. 

 Surface nearly smooth, or only obscurely ribbed in some specimens, but 

 more generally ornamented with rather strong, regular costse on each side, 

 that usually curve rather strongly forward as they approach the periphery, 

 upon which they become obsolete. 



Greatest diameter of one of the largest specimens, 2.70 inches; con- 

 vexity, about 0.60 inch. 



I have seen no specimens of this species showing either the siphuncle 

 or the septa, and it seems that none of those seen by Professor Hauer were 

 in a condition to show the nature of the septa. Mr. Gabb, however, 

 describes them as being each "composed of a dorsal and two lateral lobes, 

 which, with the saddles, are all nearly *ounded undulations ". From this 

 character of the septa and the general form of the shell, taken in connection 

 with the age of the formation in which it occurs, it is far more probable 

 that the "gullet" of its siphuncle will be found to agree with that of Professor 

 Hauer's genus Clydonites than with that of Goniatites. 



As remarked by Mr. Gabb, this shell varies considerably in its surface- 

 characters, some individuals being nearly smooth, while others have the 

 costae of the sides well developed. There are also some differences in the 

 size and the arrangement of the costse, which are sometimes proportionally 

 wider and more widely separated than in other examples, while the peripherj^ 

 is more nan-owly rounded in some individuals than in others. 



I have had no opportunity to compare this shell with typical foreign 



