392 J. F. BLAKE AND W. H. HTTDLESTON ON 



Ammonites perarmatus, Sowerby. 



The group of Ammonites passing under this name presents three 

 well-marked forms, " varieties " or " species :" — 1. The typical form 

 as represented by Sowerby, with moderately inflated, rounded, and 

 subevolute whorls, tubercles of good size, and joined by a swollen 

 rib. This occurs in the upper part of the Lower Limestones of 

 Yorkshire. 2. An inflated more involute form, with less con- 

 spicuous ribs or tubercles, most common in the Lower Calcareous 

 Grit proper of Yorkshire ; and, 3. A flatter form with the inner 

 tubercles and ribs almost obsolete, and the outer tubercles much 

 smaller and more numerous. Its chief home is in the Lower Cal- 

 careous Grit of Wiltshire and Oxfordshire ; and it is the variety to 

 the casts of which the name of A. catena was given by Sowerby. A 

 fourth form has also been met with as a single specimen at Sike Gate, 

 Yorkshire, which, as it occurs probably at a higher horizon than any 

 of the others, may serve to show that links between the perar- 

 matus group and the A. lonr/ispinus of Kimmeridgian age are not 

 wanting in the Corallian limestones. The intermediate character 

 of this fossil favours the above view. 



Ammonites psetjdocordattts, spec. nov. PI. XIII. fig. 1. 



Although Ammonitus cordatus characterizes the lower portion of 

 tho Corallian series, yet in the Upper Calcareous Grit of Yorkshire 

 we find Ammonites that are undistinguishable from it, and the 

 type was continued (through the form next to be described) into the 

 A. alternans of the Kimmeridge Clay. We need not, therefore, be 

 surprised to find it influencing Ammonites of the plicatiloid group. 

 The Ammonite we propose to call A. pseudocordattts has, indeed, 

 the ribbing of A. plicatilis, but the involution and sharpness of 

 front of A. cordatus ; so that its general aspect reminds us of both 

 groups. The following is its diagnosis : — 



Last whorl J the diameter ; inner whorls half-covered ; greatest 

 thickness of outer whorl -| the diameter, situated towards the inner 

 side of the whorl, whence to the front the slope is uniform, giving a 

 semi-acute rounded front. Aperture sublanceolate. Ornaments 35 

 ribs on the last whorl, which nearly die out and give place to about 

 96 smaller ones, which pass over the front without alteration ; dia~ 

 meter nearly 9 inches. 



In the Ironstone of Westbury. 



Ammonites cawtonensis, spec. nov. PI. XIII. figs. 2, 2 a. 



This form, though belonging to the cordatus group, is so distinct 

 as to make it worthy of a specific name. It leads us on from the 

 older forms to the newer A. alternans. 



Outer whorl f the diameter ; inner whorl half-covered ; sides of the 

 whorls between the ribs uniformly convex for |- of the breadth, 

 when they become concave on each side of the keel ; greatest thick- 

 ness of the whorls between the ribs J the diameter ; aperture con- 

 vexo-quadrate ; ornaments, 20 nearly straight ribs rising from the 

 inner edge, and running nearly radially to the middle of the whorl. 



