ARIETITES IMPENDENS. 303 



Middle-aged. — Diameter of specimen figured (PL XXII a, fig. 1) 80 millimetres ; 

 ditto of the umbilicus 26 millimetres; height of aperture 35 millimetres; width of ditto 

 at spire 20 millimetres. 



Adult. — Diameter of a specimen not figured, 100 millimetres; width of umbilicus 

 25 millimetres ; height of last whorl 45 millimetres. 



Description. — This is a very remarkable Ammonite, and combines in its structure the 

 form of Harpoceras with the lobes and saddles of Arietites. It is likewise a very rare 

 form in the Yorkshire Lias, where it has hitherto been chiefly found. In early age 

 its deep whorls have costated sides, and when 40 millimetres in diameter (fig. 4) its 

 convex sides are covered with thirty straight acute ribs, which bend sharply forwards 

 toward the margin and disappear on the area ; the keel is then acute and prominent, and 

 it has a sulcus at each side. In middle age (when 80 millimetres in diameter, figs. 1 — 3) 

 the last whorl has become very deep, and resembles a Harpoceras in form ; about twenty 

 bent ribs or undulating folds cover the sides near the umbilical suture, disappear on 

 the external half, and are bevelled away toward the margin ; the outer lamina of 

 the shell is covered with numerous fine striae, which follow the sweep of the ribs 

 and highly ornament the surface — these lines are seen only on very fine specimens 

 carefully extracted clean out of the shale ; the height of the last whorl at the aperture in 

 a specimen before me is about iiy^ths of the diameter of the shell, that of the figured 

 shell is -i- %ths ; the siphonal area is narrow, from the bevelling of outer half of the whorl ; in 

 the centre is a very acute prominent keel with two deep furrows (figs. 2, 3) ; in some good 

 specimens the keel is beautifully crenated by the fine striae of the shell passing over its 

 edge. The inner margin of the whorls overhangs the umbilical suture, hence the origin 

 of the name impendens given to this species by Young, and the outer margin terminates 

 abruptly at the outer side' of the sulcus (fig. 2). In the adult specimen, 100 milli- 

 metres in diameter, the depth of the outer whorl, and the size and prominence of the 

 keel form striking characters in the shell, which is here intact ; the amount of involution 

 is likewise much increased. 



The lobe-line (fig. 5) is very regular, the siphonal lobe is short and wide, and the 

 margins finely striated ; the siphonal saddle wide, with two festoons ; the principal 

 lateral lobe is less than the siphonal, and formed of unequal parts ; the digitations are 

 more numerous at the inner side ; the principal lateral saddle is wide, with several 

 festoons ; the lateral lobe is a little smaller than the principal, and, like it, is 

 formed of unequal parts ; the lower lateral saddle is small, and the auxiliary lobes are 

 concealed by the turns of the spire. 



Affinities and Differences. — This Ammonite closely resembles Arietites Collenotii, 

 d'Orb. They both have the physiognomy of Harpoceras, and might be readily mistaken 

 for that genus, but the structure of the lobes and saddles closely connects them with 

 Arietites, in which I have placed them. They are both rare Ammonites, and, except the 

 specimen I obtained from Robin Hood's Bay, all the others were in the cabinet of my late 



