EL. Ammonites biplex Sowerby, 1821. 
- 
CEPHALOPODA Ammonites biplex 
Sowerby, 1821. 
Original description 
(1821. — Sowerby, Min. Conch., Vol. III, p. 168). 
« AMMONITES biplex. TAB. CCXCIIL. — Figs. 1 and 2. 
« Spec. CHar. Discoid ; volutions exposed, costated ; costæ numerous, 
small, split over the front; sides depressed, aperture oblong. 
« VOLUTIONS five or six, the numerous costæ are nearly straight, elevated and obtuse, a 
little before it passes over the front each is divided into two branches, equally thick with the 
undivided rib; the oblong aperture is narrowest near the front which is round ; the external 
and internal surfaces do not difier. 
« This Ammonite occurs in a mixture of Clay with septaria and Chalk Boulders, that 
extends over a great part of Suffolk. Fig. 2 is from Barrow, given me by Mr. Barnes, of 
Norwich. » 
OBSERVATIONS 
The original of fig. 1 was refigured and redescribed, from a cast, by Dr J. von Siemiradzki 
in Palaeontographica, Vol. XLV, p. 265, PI. XXV, fig. 41, 1898, and again by me, from 
the specimen itself, in the Quarterly Journal of the Geological Society, Vol. LX, p. 57, PI. X, 
1904, when I ventured to remark that it was perhaps wisest to abandon the name altogether 
or at least to restrict it to the abnormal specimen to which it was first attached. Mr. Crick, 
of the British Museum, has since suggested to me that the structure revealed by the cross- 
section can be explained by the infiltration of calcite beneath a thin layer of the shell which 
has consequently been displaced outward. He is, moreover, of opinion that by measuring the 
small ammonite and adding the thickness of the displaced shell, accurate results might be 
obtained, by the aid of which the species might be established. The suture line is not shewn. 
Sowerby's figure is reversed. 
The specimen belongs to the genus Perisphinctes and is probably a Kimeridgian form. 
It came from the Drift of Suffolk. 
Fig. 2 is merely a fragment of another specimen. 
1904. M. Healey. 
PALAEONTOLOGIA UNIVERSALIS. — gb 
