ARIETITES 8EMIC0STATUS. 285 



Ammonites geometricus, Dumortier. Depots Jurass. du Bassin du Rhone, torn, ii, 



p. 31, pi. vii, figs. 3—8, 1867. 



Arietites semicostatus, Tate 8f Blake. Yorkshire Lias, p. 288, pi. vi, fig. 4 a 



(upper), 1876. 



Diagnosis. — Shell discoidal, slightly involute, and depressed ; whorls six, bearing 

 straight, sharp, prominent ribs, which suddenly bend round the margin and vanish in 

 the sulci ; they are separated on the sides by well-defined concave spaces ; the three 

 inner whorls are almost always smooth. Siphonal area flat; keel narrow, sharp, and 

 entire, placed between two lateral sulci ; aperture quadrate. 



Description. — This is a very common shell on the Yorkshire Coast, and a character- 

 istic type of the group Arietites. It often occurs in rocky masses of Lower Lias 

 Limestone in Robin Hood's Bay. One of those now before me contains twenty-five 

 specimens on one side, and as many more clustered together in the slab, showing well 

 the anatomy of the shell and the varieties of shape it assumes. In some the whorls are 

 round and inflated, with the ribs slightly bent ; in others the shell is flat and the ribs 

 straight. 



In early life, up to three or four whorls of growth, the sides are in general smooth, 

 like A.planorbis, or have very slight folds, as indications of rudimentary ribs ; and the keel 

 begins to show itself as an elevated ridge without sulci. At this stage of growth the 

 small shell resembles A. radians. In one of these young shells I exposed the black 

 horny Anaptychus in situ. In PI. I, figs. 7 and 8, I have represented these specimens 

 the natural size. In one (fig. 7) the whorls are smooth; the Anaptychus is seen in this 

 specimen, and fig. 8 shows the commencement of the ribbing. It was this half-ribbed 

 character of the shell which suggested to Young the specific name semicostatus. In 

 some young shells the keel has not been differentiated at the third whorl, and only then 

 appears as a prominent rudimental ridge with slight crenations on the border. 



The whorls are so entirely evolute that only the keel of the penultimate whorl is 

 embraced by that which succeeds it ; the increase of the whorls is thereby very slow, and 

 their height is proportionately greater to the width in the growth of the shell, and the 

 mouth-aperture forms in consequence a high rectangular opening. 



The evolution of the shell is as follows : at first without ribs, and then it possesses only 

 fine radial folds. About the third, fourth, or fifth whorls the ribs begin to show themselves, 

 first as flat and indistinct folds, and soon afterwards as sharp, straight, prominent elevations. 

 In a small shell with four whorls there are twenty-two ribs, in one with five whorls twenty- 

 five ribs, and in one with six whorls there are thirty-six ribs. They all proceed in a straight 

 line from the umbilical to the siphonal area, and at the marginal angle of the latter they 

 bend abruptly forward and vanish in the sulcus. The body-chamber is three fourths of 

 a whorl in length. 



The lobe line is very simple (fig. 6) ; the siphonal lobe is short, with simple lateral 

 serrations ; the principal lateral is large, with small lateral digitations, terminating in two 



