8 GEOLOGICAL MEMOIRS. 



A remarkable fact, difficult of explanation in the present state of 

 the theory of descendence, is the variation in the same direction 

 which occurs nearly simultaneously in the different genera of Am- 

 monites — namely, the complication of the sutures, and the turning 

 forward of the striae of growth upon the convex part, by which the 

 Goniatitic stage of development is converted into the Ammonitic 

 stage. In the transition stages ' ceratitiform and heterophylliform 

 sutures occur. The Triassic species of Lytoceras are all in this con- 

 dition, presenting heterophylliform (monophyllous) sutures. But 

 examples are not wanting of forms which have remained upon a 

 lower Goniatitic grade, such as Sageceras and the group of Arcestes 

 clelphinocephalits, which persists, as a Devonian antiquity, up to the 

 St.-Cassian beds. 



The new genus Pinacoceras, established for Ammonites Metternichi 

 and some allied forms, is characterized as follows: — Animal un- 

 known ; shell very narrow, with a high mouth, smooth, often 

 having folds and tubercles ; body-chamber occupying |— | of the last 

 whorl, with short lobes projecting from the couvex part; muscular 

 impression commencing on the convex part at the anterior extremity 

 of the body-chamber near the aperture, and descending upon the 

 lateral parts to the concave part at the hinder extremity of the 

 body-chamber ; surface of attachment of the mantle consisting of 

 punctiform or striiform elevations ; wrinkled layer of separated 

 radiating striae projecting as a lobe beyond the margin of the arjerture 

 upon the convex part of the preceding whorl ; embryonic nucleus 

 vesicular ; lobes consisting of three different groups : — a variable 

 number of adventitious lobes formed by the removal of the siphonal 

 tubercles, and increasing by division ; three deep main lobes ; and a 

 variable number of auxiliary lobes. 



The new genus Sageceras, founded for Ammonites Uaidingeri and 

 its allies, approaches Pinacoceras by its flat, discoidal form, and the 

 great number of lobes lying outside the line of projection of the 

 preceding whorl. An essential distinction is furnished, however, by 

 the process of the concave part, which is distinctly marked by the 

 direction of the striae of growth. There are further differences in 

 the peculiar form of the lobes and in the different nature of the 

 wrinkled layer, which is granular in Sageceras as in Nautilus. 



In Arcestes . the varices prove to be really remnants of former 

 apertural margins, and must not be confounded with the muscular 

 impressions which frequently occur in Pinacoceras. These internal 

 ridges of the shell occur close behind the margin of the aperture in 

 those forms of Arcestes in which the shell is not inverted at the 

 margin. There is consequently only a morphological difference 

 between varices and contractions, the latter being produced by the 

 inversion of the shell at the margin of the aperture. 



It is a remarkable fact that of all the fossils of the K" oric division 

 of the Hallstatt limestone and of the Zlambach beds, not a single 

 species has hitherto been found beyond the region of the north- 

 eastern Alps (Salzburg, Upper and Lower Austria, and Stvria). 



[W. S. D.] 



