156 INFERIOR OOLITE AMMONITES. 



tendency to become concave.^ Inclusion rather more than one-half ; but showing 

 a tendency to decrease in the adult. Umbilicus small and in youth somewhat 

 deep ; shallower in the adults. Aperture more or less acutely sagittate. 



The species was first figured by Quenstedt {he. cit.) who, then and since, has 

 regarded it as a compressed involute variety of Am. radians (see later, p. 188). 

 Subsequently Oppel, quoting Quenstedt' s original figure, gave the species the 

 distinctive name of Am. Eseri; and afterwards he figured a specimen in 

 the ' Palaeontologische Mittheilungen.' This figure is perfectly accurate except 

 in one respect, namely, that the carina is drawn much too small. Oppel evidently 

 had to deal with a cast which possessed a carina chipped and imperfect — as is the 

 case in many specimens — all round the whorl. This fact, however, is not shown 

 in the figure — the carina has been drawn with a perfect edge — and sufficient 

 allowance has not been made for height. Quenstedt's figure in " Die Cephalo- 

 poden " is far more accurate in this respect, and indicates the partition-band 

 and the correct position of the siphuncle with regard to the hollow-carina. My 

 figures, which show the chipped carina drawn as it actually is, will explain how 

 the mistake in Oppel's figure could have arisen. 



Oppel's figure represents a much more compressed specimen than Quenstedt's ; 

 yet presumably, as the latter was first quoted by this name, we must regard it as 

 the type of Oppel's species. These two forms — the thick (a) and thin (/3) — are 

 the two usual varieties of the species ; but we also have (7) a compressed form 

 with large umbilicus (Plate XXV, fig. 7), and (S) a form with unusually coarse 

 irregular ribs (Plate XXVI, figs. 1, 2). 



The form a approaches If. occidentalism but has a sharper ventral area and a 

 smaller umbilicus. 



The form y, on the other hand, agrees in the size of its umbilicus with 

 H. occidentalis, but is much more compressed, and has a sharper ventral 

 area. 



The form S is distinguished by its very coarse ribs. 



A fragment of this species was figured by Dumortier as Ammonites exaratus 

 {op. cit,, pi. xii, fig, 4). Figs. 1, 2 on the same plate appear to me to be the 

 form (3 of this species without any test, and with the ventral area deprived of 

 its hollow carina. The species which he has figured as Am. Eseri is not this 

 species at all, but is a variety of Grammoceras fallaciosnm (p. 204). 



Bayle (loc. cit.) has also figured a variety of Gram, fallaciosum under the name 

 Grammoceras Eseri. Vacek, too, has failed to correctly interpret this species. 

 His Earpoceras Eseri^ belongs, as the strongly-drooping inner part of the suture- 



1 This ia an interesting foct, especially in connection with ray remarks at p. 90. 

 " "Oolithe von Cap San Vigilio ;" 'Abh. der k. k. geol. Eeichsanstalt,' Baud xii, No. 3, pi. ix 

 fig. 5. 



