450 THE LIAS AMMONITES. 



to arrive at the truth. One small specimen, 5 millimetres in diameter, was costated and 

 carinated like the outer whorl of the same species 40 millimetres in diameter. So that 

 both the form, ornamentation, and figure are retained with very little change through life 

 in this species. My learned friend, Professor Quenstedt, with his usual accurate eye for 

 the discrimination of Ammonite forms, has described the following varieties of Am. 

 radians : 



a. Radians depressus, which is identical with Ammonites striatulus, Sowerby. 



(3. Radians compressus, resembling Am. Capellinus, Reinecke; its outer whorl is much 

 higher than in Am. radians depressus. 



y. Radians quadratus, Quenst., of the type of Am. radians depressus ; the keel has 

 two lateral grooves on each side as in Harp, bifrons. 



$. Radians costula, Rein. 



e. Radians comptm, Rein., an intermediate form between Reinecke's Ammonites 

 radians and Quenstedt's Ammonites ammonius, to which Harpoceras opalinum and 

 Harpoceras Murchisoni appertain. 



The septa are well displayed in figures 1 and 4, PI. LXIV. They are symmetrical 

 and are on each side divided into three lobes and three saddles, formed of single parts. 

 The siphonal lobe is narrower and shorter than the principal lateral, and ornamented on 

 each side with three digitations and two terminal divergent branches. The siphonal 

 saddle is as large as the principal lateral lobe and divided by an accessory lobe into two 

 unequal portions, the internal portion being the largest, and both terminate in many large 

 unequal-sized folioles. The principal lateral lobe is large and ornamented on each 

 side with five short and three elongated terminal branches, with numerous points to 

 each branch. The lateral saddle is one third less than the principal lateral lobe and ends 

 in large leaf-like expansions. The inner lateral lobe is not one half the size of the 

 principal, its sides have several irregular digitations ; besides these regular lobes there 

 are two or three auxiliary lobes which are both short and irregular in figure. 



The morphology of this species shows that at the diameter of 5 millimetres the keel 

 and ribs were developed, the latter chiefly on the area first, afterwards appearing on the 

 sides. At 24 millimetres diameter there were nearly forty ribs ; at 43 millimetres 

 diameter fifty-four ribs ; at 70 millimetres diameter there were seventy ribs ; and at 

 128 millimetres diameter the shell had ninety-four ribs, so that the number of the ribs 

 of Harpoceras radians was in a direct ratio with the diameter of the shell. In the 

 large German specimen, belonging to the British Museum Collection (PI. LXIV, figs. 

 5, 6, 7), which attains 135 millimetres in diameter, there are ninety-six ribs. 



Affinities and Differences. — Harpoceras radians very much resembles Harp, ser- 

 pentinum both in form and ribbing. Its whorls, however, are not so high, and its ribs 

 are only flexed and not bent in the middle ; the sides are likewise convex, more involute, 

 and not truncated at the inner border. 



Locality and Stratiyraphical Position. — This Ammonite attains a full development in 



