22 
NAUTILOIDEA. 
upper part, so as to give them a conical appearance when seen in 
section. There are no traces of annulatious upon the shell. The 
test, which is very thick in adult specimens near the aperture, is 
ornamented with subregular striae of growth, which form a sinus 
both upon the ventral and the dorsal sides of the shell. On each 
side of the aperture, close to the convex side, the test forms an 
expansion or prominent wing-like fold. These aliform projections 
strikingly resemble those developed upon the shell of Gyroceras 
alatum, Barr. (pi. xliv.). There is a second pair of projections at 
the base of the body-chamber in TrocJioceras Davidsoni, just as 
there is upon the body-chamber of G. alatum. They have not 
been seen upon the septate part of the present species, though, 
as observed by M. Barrande, their absence may be due to bad 
preservation. 
Remarlcs. Trochoceixis Davidsoni is distinguished from the other 
smooth species by its size, by the subquadrangular form of its 
transverse section, by the great relative size of the body-chamber, 
and by the aliform projections with which it is ornamented. But 
for its asymmetry this species might have been referred to Gyro- 
ce 7 'as, to which it may be perhaps a connecting form through G. 
alatum. 
Horizon. Etage E ( = Downtonian). 
Locality. Konieprus, Bohemia. 
Fairly well represented in the Collection. 
Trochoceras secula, Barrande. 
1848. Cyrtoceras secula, Barrande, in Haidinger’s Berichte iiber die 
Mittheil. von Freund, der Natiu’wiss. in Wien, Band iv. p. 208. 
1867. Trochoceras secula, Barrande, Syst. Sil. de la Boheme, vol. ii. 
pt. i. p. 127, pi. XXX. If. 1-6. 
1877. Trochoceras secula, Barrande, ibid. Suppleni. et Serie tardive, 
p. 239, pi. dx. if. 1-5. 
Sx). Char. Shell scarcely attaining a complete volution, and but 
slightly asymmetrical. The enrolment of the whorls is dextral. 
The section is elliptical, the slight!}^ smaller extremity correspond- 
ing with the concave side of the shell. The ventro-dorsal is to the 
transverse axis as 4 : 3. The shell increases its diameter about three 
times in the course of the last volution. The length of the body- 
chamber equals | that of the entire shell. The plane of the aper- 
ture is slightly inclined towards the concave side. The septa are 
distant from each other 1' the ventro-dorsal diameter, and their 
convexity is equal to J of the same. The siphuncle is placed close 
to the concave border, but without touching the test, its elements 
