182 GENESIS OF THE ARIETIDA. 
Coroniceras trigonatum, Hyarr. 
Plate VI. Fig. 3. Plate VIL. Fig.1. Summ. Pl. XIL. Fig, 15. 
Aster. trigonatum, Hyatt, Bull. Mus, Comp. Zodl., I., No. 5, p. 79. 
Amm. Brooki, Zunr., Verst. Wiirt., p. 26, pl. xxvii. fig. 2. 
Amm. Brooki (Riesenbrooki), Quenst., Der Jura, p. 68. 
Amm. Crossi, QuENST., Amm. Schwiib. Jura, pl. 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 pile tuberculated, as in Cor. lyra. 
After this the dorsum increases more rapidly, and the whorl gradually assumes 
the trigonal form. The tubercles and genicule become atrophied, the pile 
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 (multicostatus 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 pile at the same 
age than in Cor. ra. 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. Riesenbrooki of Quenstedt, 
and the less involute are the normal Amm. Brooki of most German authors. 
Wright’ describes the Stuttgardt and Tiibingen specimens, but considers 
them identical with Cor. Gmuendense and his Arietites Cross’, —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. 
