STEPHANOCERAS RAQUINIANUM. 479 



split up into two or three smaller costse, which pass across the outer margin and join 

 those of the opposite side. Siphonal area large and convex ; aperture depressed, sub- 

 quadrate. 



Dmensio7is. — Transverse diameter 45 millimetres; height of last whorl IS milli- 

 metres; width of umbilicus 15 millimetres; height of aperture 18 millimetres; width of 

 aperture 19 millimetres. 



Description. — The shell of this species is very thick ; the whorls are one half involute 

 and depressed, much wider than deep ; the sides are inflated and costated, with from 

 twenty-two to thirty-two short, straight ribs, which commence at the umbilicus and 

 terminate at the lateral border of the siphonal area, either iil a small blunt tubercle or a 

 fasciation of the ribs ; from these points two or three minute costse pass across the area 

 and unite with their fellows from the other side of the shell ; besides the small costae 

 proceeding from the tubercles or fasciated group there are other simple costse which pass 

 between the ribs from the umbilicus and the area, and others between the costae on the 

 area and contribute to the increased costation which adorns the broad siphonal area of 

 this beautiful form. 



The siphonal area is very broad, depressed, and convex, and closely covered with fine 

 transverse costse. The spire, composed of depressed whorls, is wider than high, and 

 embraces half of the preceding whorl. The aperture is semilunar and depressed, 

 convex above and concave below, where it receives the return of the spire. 



The septa are symmetrical, and form on each side two lobes of single elements and 

 two large saddles. The siphonal lobe is as long and much wider than the principal 

 lateral, and ornamented on each side with three branches, of which the lower one has 

 five points. The siphonal saddle terminates in three unequally festooned folioles. The 

 principal lateral lobe terminates in three branches, which are slightly digitate. The lateral 

 saddle is neither so deep nor wide as the siphonal, and ends in four shallow folioles, 

 divided by an accessory lobule ; the auxiliary lobes are rudimentary. 



Prof. d'Orbigny^ investigated the morphology of this species, and found that at the 

 diameter of 20 millimetres it possessed nineteen costae; at the diameter of 35 millimetres 

 it possessed twenty-seven, but at the maximum height it had forty-eight. The Professor 

 stated also that in youth it had always an intermediate rib between the fasciated pair, 

 whilst the bifurcated ribs alone prevailed in adult forms. He noticed that when the 

 test was preserved the costae were prominent, but they stood out very feebly on the 

 mould, and sometimes disappeared altogether from the area ; further, that when the 

 aperture possessed the termination of the oral appendage the mouth of the shell had a 

 much smaller diameter than the rest of the whorl. 



Affinities and Differences. — This shell differs from Stephanoceras Brauniaiium (PI. 

 LXXXVI, fig. 4) in having inflated sides and a convex area ; in the ribs on the sides 

 being shorter, and in the general costation being neater. 



^ ' Paleontologie Fran9aise, Terr. Jurassiques,' torn, i, pp. 333. 



