530 MU. T. II. WITIIEIIS ON 



the cai-in?e of the two species >S. arcuaiuvi and aS'. irilineaiuvi, and 

 pointed out the possibiUty of their pi'esence in iS. accumulcUam. 

 The intraparietes in these species, instead of forming a thin wall 

 on each side of the carina as in the form represented by text- 

 fig. 64, 5, are bent inwards almost at right angles, and the npper 

 regions of their inner margins meet to a greater or less extent 

 (see text-fig. 64, 3 b, 4 b); the upper part of the valve is solid, and 

 must have projected freely to the extent indicated by the meeting 

 of the intraparietes*. The peculiar development of the intra- 

 parietes in the carinse of these species is therefore of importance 

 as showing a development of the carina distinct from that in 

 which the intraparietes form a thin wall on each side of the 

 carina. 



Three types of carina, all having an apical umbo, were there- 

 fore already developed amongst these early forms of ScalpeUum, 

 and the geologically oldest of these more closely resemble the 

 carina of PoUicipes, from which Sccdpellum is considered to be 

 derived. These are (1) represented by S. simplex (text-fig. 64, 1) 

 from the Aptian (Lower Greensand), which has no intraparietes, 

 the tectum being flatly-arched transversely, the parietes bent 

 almost at right angles to the tectum and not extending to the 

 basal margin ; this type is distinguished from the PoUicipes type 

 of carina only in the parietes being separated from the tectum 

 by a distinct angle ; (2) represented by S. accumulaUim (text- 

 fig. 64, 2) (Aptian, Lower Greensand), S. arcuaUmi (text- 

 fig. 64, 3) (Albian, Gault), and S. trilineatum (text-fig. 64, 4) 

 (Cenomanian, Grey Chalk), which have intraparietes, these parts 

 being bent inwards almost at right angles and joining, the upper 

 part of the valve being solid and projecting freely to a consider- 

 able extent ; in the solidity and free projection of the upper part 

 of the valve this type is allied to PolUcijies ; (3) represented by 

 S. hastatum (text-fig\ 64, 5) and other species from the Ceno- 

 manian which have intraparietes also, but are characterized by 

 these parts forming a thin wall on each side of the carina, the 

 apex of which projects freely, slightly, or not at all ; this latter 

 type is more typical of the genus Sccdpellum, and, owing to the 

 upward growth of the intraparietes in some forms, subsequently 

 gave rise to the species with an angularly bent carina having the 

 umbo in a subcentral position, a tj'^pe which is not known below the 

 Upper Senonian. The only species with an angularly bent carina 

 known from the English Chalk is S. darioiniammi Bosquet f, but 

 this has an early specialized form of carina in which the upward 



* A somewhat similar development of the intraparietes can be seen in the carinie 

 of (S. maximum var. cylindraceum Uavwin (1851, p. 33, pi. ii. fig. 2) from the Upper 

 Senonian {Belemmtella miicronata-zoiie) of Norwich, Norfolk, and S. soUduhim 

 Steenstrup (1839, Kr^yer, Naturhist. Tidsskrift, Bd. ii. p. 412, pi. v. figs. 14, 14* ; 

 Darwin, 1851, Pal. Soc. Monogr. Foss. Lepadidse, p. 42, pi. i. tig. 8) from the Upper 

 Senonian of Kjuge, Scania. 



f J. Bosquet, 1854, Les Crust. Foss. du Terrain Cretac^ du Duchc de Lirabourg, 

 p. 46, pi. iv. figs. 6-12 ; T. H. Withers, Jan. 1911, Gaol. Mag. dec. v. vol. viii. 

 p. 23, figs. 3-4. 



