182 GENESIS OF THE ARIETID.E. 



Coroniceras trigonatum, Hyatt. 



Plate VI. Fig. 3. Plate VII. Fig. 1. Slimin PI. XII. Fig. 15. 



Aster, trigonatum, Hyatt, Bull. Mus. Conip. Zool., I., No. 5, p. 79. 

 Amm. Brooki, Ziet., Verst. Wiirt, p. 26, pi. xxvii. fig. 2. 

 A mm. Brooki (Riesenbrooki), Quenst., Der Jura, p. 68. 

 Amm. Crossi, Quenst., Amm. Schwab. Jura, pi. xiv. fig. 6. 

 Amm. nudaries, Quenst., Ibid., fig. 5? 



Locality. — Aalen 



Great size is a characteristic of this species. One specimen was 380 mm. in 

 diameter, and another measured 503 mm. Quenstedt describes one from En- 

 dingen 700 mm. in diameter. 



The young were seen only from the side, but the following characters could 

 be made out, even from this point of view. The form of the whorl in trans- 

 verse section at an early stage is evidently quadragonal, since during the first 

 four or five whorls the sides are flat and the pilse tuberculated, as in Cor. lyra. 

 After this the dorsum increases more rapidly, and the whorl gradually assumes 

 the trigonal form. The tubercles and geniculse become atrophied, the pilaa 

 are merely lateral folds, and the channels very shallow by the time the shell 

 reaches the second quarter of the sixth or seventh volution. On the eighth 

 volution the channels are represented only by smooth inflected zones on either 

 side of the keel. 



The sutures on the latter part of the sixth or seventh whorl have abdominal 

 lobes, which are one half longer than the superior laterals, the inferior lateral 

 saddles one half deeper than the superior laterals. The lobes and saddles are all 

 exceedingly broad, and the outlines very complicated. After this period the 

 sutures degenerate, the abdominal lobe decreases in length, and the lobes and 

 saddles, though they retain their complicated marginal inflections, become pro- 

 portionally broader. 



A fine suite of specimens in the Museum of Stuttgardt, from the Geometricus 

 bed, show that the species is very similar to Cor. lyra {muUkostcdus of German 

 authors). The whorl, however, has a broader dorsum, and is larger, and the 

 volutions are less numerous. There are also fewer and stouter pilae at the same 

 age than in Cor. lyra. There are two varieties included under this name. One 

 is a smaller form, with premature development of senile whorls, etc., and has 

 broader whorls ; the other is the normal form, having slower development of the 

 adult and senile characters, and this alone has been figured. 



The more involute shells of this species are the Amm. Riesenbrooh ''of Quenstedt, 

 and the less involute are the normal Amm. Brooki of most German authors. 



Wright 1 describes the Stuttgardt and Tubingen specimens, but considers 

 them identical with Cor. Gmucndense and his Arietites Crossi, — a mistake arising 

 from not having observed the differences in the umbilicus due to the number 

 and shape of the whorls, which are less numerous and stouter than in Gmuen- 

 dense. Quenstedt appears to have been led into a similar error, possibly through 



1 Lias Amm . p 284. 



