HYPERLIOCERAS DISCOIDEUM. 99 



very large, much-raised carina, which occupies in breadth nearly half the ventral 

 area. Inner margin concave and overhanging; umbilicus small and consequently 

 much contracted ; aperture triangular. 



j3. Whorls flatter, sloping from middle only ; inner margin concave, sloped ; 

 aperture sagittate. 



Under the name Ammonites discoideus, Quenstedt has united several different 

 forms which I can scarcely agree to place in the same species. Some of them 

 appear to have much greater resemblance to Hyperlioc. discites ; but no front view 

 is shown us. Selecting, however, his fig. 1 as a very distinct form of the genus 

 Hyper lioceras, I notice that there are in my possession certain specimens which 

 approach it very closely ; and, though they do not exactly tally with it, yet they 

 must come under this name. With the type they may be noticed as follows : 



TJie Type. — Quenstedt's form (restricted as above mentioned) : its exact counter- 

 part has not yet been found in England. 



a. (Plate XIX, figs. 3 and 4.) Differs from the type in having a more elongated, 

 and more acutely triangular, aperture, and in possessing an overhanging inner 

 margin. 



|3. (Plate XIX, figs. 1 and 2.) Differs from the type in not having an exactly 

 triangular aperture, and in being compressed on the inner area. It resembles the 

 type in having a slightly sloped inner margin, and in this it differs particularly 

 from a. 



The triangular or subtriangular aperture, and the narrow external edge, dis- 

 tinguish this species from the others ; and a larger umbilicus separates it from 

 Hyperlioc. Desori. Its general shape, and especially its narrow external edge, 

 recall the form of its ancestor Psendolioceras compactile. This external edge 

 carries a carina higher, and also rather narrower, than that of any other species 

 of the genus. The test on the outer edge of the carina is of considerable 

 thickness, and the carina altogether is very prominent ; in fact, no species of the 

 solid-keeled Ammonites that I know possesses a larger carina than this one. 

 This interesting fact will be important when we come to deal with certain hollow- 

 carinate forms, which in shape resemble this species very closely, and have a carina 

 as prominent, but hollow. 



On account of its peculiar shape, and also because certain specimens possess 

 on the test faint longitudinal strige and ridges, the form a is frequently labelled 

 " Am. Truelli." This is a very serious mistake, which the slightest study of the 

 suture-lines would detect. 



The species occurs in the Co7icavum-beds at Bradford Abbas and Halfway 

 House, Dorset, but is certainly very scarce. I have seen no young specimens 

 which I could refer here with certainty. Incomplete specimens of the form a 

 sometimes measure as much as eight inches in diameter. This form is very 



