PLACENTICERATID &. 201 
with those of the opposite side; the outer row is more numerous than the 
inner, and there are slight indications of bifurcated ridges of costae con- 
necting them on the cast. The ventral tubercles are irregularly alternate 
with the second row and there may have been ridges bifurcating more or 
less between these, but there are no indications of these on the cast. 
It is very like Morton’s species, but the gerontic stage begins later and 
the increase of the ventro-dorsal diameters is much more rapid. The width 
through the umbilical shoulders is greater at the same age and the lateral 
zones flatter and more convergent, owing to the greater prominence of the 
umbilical shoulders. The ventral lines of tubercles are more elongated, 
not so close together, and quite different, and the second inner line of tuber- 
cles is less prominent and nearer to the vertical lines. The inner lines ot 
tubercles do not appear until the ephebic stage and are at first very minute 
but rapidly enlarge in the remainder of the ephebic and gerontic stage, 
disappearing suddenly before the outer ones at the beginning of the para- 
gerontic substage. They recede from the umbilical shoulders outwardly m 
the parephebic and gerontic stages and have an inner costation or ridge 
inclined apicad. 
The outer line of tubercles disappears in the paragerontic substage 
immediately after the inner line. The ventral lines of tubercles disappear 
on the cast in the metagerontic substage. These tubercles are present on a 
bit of the thick ventral shell in the ephebie stage. These are almost linear, 
alternating and widely separated, and border a slightly concave ventral 
zone, which is, however, flat upon the cast at the same age. The ventral 
zone continues well defined and flat upon this cast until quite close to the 
aperture in the extreme of the paragerontic substage. The contraction of 
the gerontic volution is very marked, beginning, even in the parephebic 
substage, before the gerontic septa appear and apicad of the base of the 
gerontic living chamber. 
The sutures have the aspect of those of syrtale, but the outlines are 
more complex and the third lateral lobes longer and more pointed. 
This does not appear to be due to greater age, but correlates with 
the larger size and other differences in the form and development of this 
specimen. There are eleven lobes and twelve saddles on each side in ana- 
gerontic septa; the innermost saddle is narrow and apparently entire, all 
the remainder divided and bifurcate except the first laterals. In these the 
