142 



THE GEOLOGIST. 



large^ considerably longer tlian the superior lateral one. All 

 the species are peculiar to the Cretaceous rocks {A compres- 

 sissimus, Didayamis, from the Neocomien ; A. Quercifolius, 

 from the Gault; A. Largilliertianus, Beaimiontiams, Sartou- 

 sianus, Vibrouyeanus'^, Ferandianus, Lafresnaiganus, Catillus 

 from the Chloritic Chalky and Upper Green Sand). 



13th Group, The Armati, Buch. — Shell with the whorls 

 square, furnished, on the sides of the back, with a range of 

 salient tubercules, and, on the sides, with one or two others. 

 Back wide, square, joining the sides at an acute angle. Septa 

 composed of lobes formed of unequal parts, the saddles formed 

 of equal parts. Dorsal lobe longer or equal to the superior 

 lateral lobe, the latter placed in the middle of the sides, and 

 always narrower than the dorsal saddle. All the species are 

 from the Jurassic formation, and principally from the upper 

 beds [A.perarmatus, Bakeries, longispinus, &c.) 



14th Group. The Angulicostati, d^0rb.—>S7ze// thick, 

 whorls almost round, but marked, on each side of the back, 

 with slightly salient parts, which render the latter almost 

 square. Back much narrower than the sides. The elevated, 

 alternate ribs, pass over the back. Septa composed of lobes, 

 formed of unequal parts, and saddles generally equal. The 

 dorsal lobe shorter than the superior lateral one ; the auxi- 

 liary lobes oblique towards the umbilicus. They belong to 

 the lower Cretaceous rocks ( A . anglicostatus, Martinii, eras- 

 sicostatits, Gargarsensis, Coimaelianus, Deshayesi, from the 

 Neocomien ; A, Milletianus, Puzosianus, fissicostatus^ from 

 the Gault, or Lower Green Sand). This group only differs 

 from the Planulati from being characterized by the square 

 back. 



* This species is only placed here on account of its external characters. 

 It is quite abnormal, on account of the singular form of its undivided 

 lobes. 



