Group 7. — Finibriati, U Orhigny. 



Sub-genus Lytoceras, Suess. ( Thi/sanoceras, Hyatt.) 



Ammonites Filicinctus (N. Sp.) 



Plate II., Figs. 2, 2<i, 2b, 2c, and 3. 



Shell composed of many, slightly involute whorls, which are nearly 

 circular in section when half grown, but which become a little com- 

 pressed laterally with age. Sm'face ornamented with minute, crowded, 

 transverse, raised lines, and with numerous periodic furrows or con- 

 strictions. 



Whorls about six or seven, two-thirds or more of the inner ones being- 

 exposed. In a specimen (Plate II., fig. 2) which measui*es about an 

 inch and a quarter in diameter, the outer whorl is rounded, but com- 

 pressed slightly and obliquel}" at the sides, while the downward and 

 inward curve towards the sutures is abi-upt and sub-convex. At this 

 stage of growth the aperture is broader than high ; its outline is almost 

 circular, but the base is shallowly emarginate. In a fragment of a 

 much larger and undistorted specimen, the oblique flattening of the 

 sides is more decided, and the outline of the aperture is rather more 

 elliptical. 



The true proportions of the umbilical opening can be seen onlj" in the 

 small specimen i-epreseuted at Plate II., fig. 2, the others are either 

 imperfect or crushed out of shape. In this individual, the width of 

 the umbilicus (the margin of which is rounded and indefinite) is not 

 quite half of the entii-e diameter. 



The whole surface of the testis encircled by fine, transverse, thread-like 

 striae or 7-ai-^pd lines, which are scarcely visible to the naked eye and 

 give to the shell a silky lustre. The striae are nearly straight on the 

 periphery, but flexuous on the sides and inner margins of the whorls. 

 They are parallel, simple, and nearly equidistant ; never either crenate 

 or bifurcating. Besides the stria3, the whorls are girdled by periodic, 

 transverse furrows or constrictions, which occur at irregular and some- 

 times distant intervals, in half-grown shells. In the outer whorl of large 

 specimens, these furi'ows become more numerous and regului-ly disposed, 

 so that when viewed at a little distance, the 6culj)tm-e near the mouth 

 seems to consist of broad flattened ribs, separated by deep, though 

 comparatively narrow, grooves. It requires a closer inspection to detect 

 the thread-like striae on the surface of each rib, but although they (the 



