266 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. Ixxvii, 



Palseontological Note. 



The most abundant and characteristic forms in the foregoing list are 

 Ammonites of the acutocarinate falcicostate type. These constitute a well- 

 defined gToup which appears to have a wide distribution in the Western 

 Hemisphere, having been described from the United States, Mexico, Peru, 

 and Brazil, Avhile in Europe it is represented by allied forms such as 

 Schlcenhachia royssiana (D'Orbigny). 



The existing names of the species united here are not sufficient even to 

 cover all the forms already figured, and numerous others still remain to be 

 described. Dr. L. F. Spath, to whom I am indebted for much valuable 

 assistance and advice, is, however, at present engaged on a much-needed 

 revision of the group ; and when this is completed many of the difficulties of 

 nomenclature will doubtless be solved. In the meantime, my remarks on the 

 Peruvian examples must be regarded as purely tentative ; for, as Dr. Spath 

 has suggested, many forms may prove to be mutations characteristic of 

 various horizons, and, until the zonal sequence has been fully worked out, 

 any definite specific determinations might only lead to further confusion. 

 Unfortunately, the figures of many types described by earlier writers are 

 unsatisfactory, and for this reason the correct identification of the various 

 forms present in my collection, representing probably at least six distinct 

 species, is by no means easy. 



Another factor that must also be taken into consideration, to which, in my 

 opinion, sufficient importance has not hitherto been attached, is the great 

 dissimilarity that exists in some forms between specimens in Avhich the outer 

 shell is preserved and those in which it has been removed, exjDosing the 

 underlying cast. My meaning will be best understood by reference to the 

 specimen figured in PL XVI, fig. 3. In fig. 4, which is a jDortion of the same 

 specimen (natural size) with the shell preserved, the ribs are seen to be 

 comparatively broad and flat, and to be separated by narrow sulci ; the 

 sui'face in fact being not unlike 'Bedford cord.' When, however, as near the 

 broken end of the last whorl in fig. 3, the outer shell is missing, the ornament 

 presents quite a different appearance. The pseudo-ribs, which are in reality 

 casts of internal corrugations, are much more strongly defined, and are 

 separated by furrows of equal width (compare also the figured cast of another 

 specimen, PI. XVI, fig. 1). 



It is thus, I think, clear that specific descriptions based on the cast may 

 differ radically from those based on the complete shell. That the former 

 method has been employed in many cases, "without any statement to that 

 effect, or reference to the state of preservation, is shown by the clear defini- 

 tion of the septal suture-lines in the figaired examples. These, of course, 

 would not be visible if the outer shell had been preserved. 



The earliest-kno"^\Ti species belonging to this group is Ammonites pern- 

 vianus described from Peru in 1839 by C. L. von Buch.^ In 1858 this species 

 was again figured by J. Marcou from the Elm Fork River (Texas) - and iden- 

 tified by him with A. acxitocarinatus of Shumard. In his description {loc. cit.) 

 the ribs are stated to be simple, and much larger than the intervals that 

 separate them ; and, since no indication of suture-lines appears in the figures, 

 it is probable that he was dealing with specimens in which the shell was 

 preserved. The type-figure of A. aciUocarinafus Shumard-^ is, however, very 

 unsatisfactory, and has led to several different interpretations being given 

 of it by later authors. The specimen has the appearance of being in the form 

 of a cast, and bears not the least resemblance to that figured by Marcou. 



Another species from Peru is A. carhonarius Gabb, collected by Eaimondi 



1 See BibHography, § IV, No. 38, pp. 5-6. 

 ^ ' Geology of North America ' p. 34 & pi. v, figs. 1, la, 16. 

 ^ B. F. Shumard, ' Natural History of the Red River of Louisiana ' App. E, 

 Paleontology (1853) p. 209 & pi. iii, fig. 1. 



