MOLLUSC A. 
13 
is swelled and abruptly rounded near the umbilicus, then flat 
on the sides, and bluntly angular between these and the back. 
This is flat, and has a greater width than our crushed speci¬ 
men would at first indicate. The measures, so far as can be 
ascertained, would be as follows :— 
Section broadly sagittate : its length 15. Breadth at the 
thickest part, near the umbilicus, 16. Breadth at the inner 
edge of the umbilicus, i. e., of the preceding whorl, 7. 
Breadth of the back 12. The surface is destroyed. 
The septa are very oblique forwards, bent sharply over the 
umbilical slope, then sweeping forward in a low curve over the 
flattened sides, and nearly direct across the back. They are 
closely placed, very much as in the carboniferous species. 
Locality .— (1777) Gunesgunga; in a boulder. Formation 
somewhat uncertain. 
CYRTOCERAS CENTRIFUGUM. 
Plate 2, fig. 1. 
C. ventricosum, ad latera compression, conicum vix curvum. Os 
contractual ?—17 lineas longo—13 lato. Sipliunculus externus. Septa 
concava, ad latera arcuata, nec obliqua, intus multo confertiora. 
Slightly curved, ventricose (contracted toward the last cham¬ 
ber?), the section not much compressed laterally, broad, oval, 
17 lines by 13, the siphon quite external. The septa are rather 
close, and concave; considerably arched backwards (but not 
oblique) on the sides, and therefore projecting forwards on the 
ventral and dorsal aspects. Last chamber . . . . ? 
This has the aspect of a Phragmoceras much more than of a 
Cyrtoceras, and was for some time mistaken for one. But the 
external position of the siphon decides the genus, for in Cyr¬ 
toceras it varies from a dorsal to a ventral position, as has been 
observed by Barrande; while in Phragmoceras it is always 
internal. 
The ventricose form (possibly contracted at the mouth, for 
this is not quite certain, and may be due to imperfect preserva- 
