482 



MOLLUSCA 



PHYLUM VI 



uneqiial, slightly excavated adductor scars, corresi^oiiding to two broad myophores in 



the Upper valve. Middle and Upper Cretace- 

 ous ; Europe and Texas. 



Subgenera : Lapeirousia Bayle. The smootli 

 bands correspond internally to tAvo prominent 

 tubercles. Synodontües Pirona. Has the two 

 teeth of the upper valve fused. 



Sphaerulites Delam. {RadioUtes, Birostrites 

 Lam. ; Jodamia Defr. ; Dipilidia. ? Agria 

 Math.) (Figs. 779- 

 781). Externally 

 like Radiolites, but 

 without the two 

 bands; valves with 

 a re-entrant sinus 

 between the teeth, 

 which. fit into sepa- 

 rate pits {dj d'), 

 usually joined by a 

 ridge with the 



Fio. 781. 



Fig. 780. 



Sphaerulites foliaceus Lam. Carentonian ; Ile 

 d'Aix, Charente. A, Sinus of the hinge, a, a', 

 Portion of the outer shell layer Anterior and posterior adductor scars ; d, d', 

 Anterior and posterior grooved sockets for tlie 

 processes of the upper valve ; x, x', Empty spaces 

 of the ligament pits ; y, Cavity at the inner end 

 of the Sinus. 2/3 (after Goldfuss). 



of the lower valve of a Sphaeru 



Utes or Radiolites showing the 

 hin^Pr marfrin of ^^^8® hollow prisnis. Cretace- 

 mner margin 01 ^^^ . jj^^^.^ Gargano, itaiy. Vi- 



the sinus ; the two 



depressions (Fig. 781, x, x) next the sinus were shown by Pethö to have been the 

 seat of a ligamentary connection between the valves ; the adductor scars {a, a) resemble 

 those of Radiolites. Widely distributed in the Middle and Upper Cretaceous. 



The supposed genera Diinlidia, Birostrites and Jodamia are based on internal 

 moLilds of Radiolites. The visible submersion of the ligament in some Radiolites 

 enables us to understand how the stages shown by Hippurites have arisen. 



Family 14. Hippuritidae Gray. 



Shell suhstance of two layers, the external porous, grooved and punctate ; the inner 

 lacunary and prismatic ; exterior with sutures corresponding to an " anal " and 

 '' hranchial " inßection, and sometimes with a ligamentary suture ; clithrum formed of 

 two processes in the free valve, the adductors attached to myophores ; fixed valve with one 

 thin laminar process ; the adductor scars excavated, the anterior adductor duplex, forming 

 distinct scars. Cretaceous. 



Hippurites Lam. (Figs. 782-786). Lower valve cylindro-conic, sometimes a metre 

 in length, attached by the apex, smooth or longitudinally ribbed, wdth three furrows 

 bounding two " columns," or columnar areas, extending from the apex to the upper 

 margin {A, B, C). Upper valve depressed, conic, with siib-central umbo, usually with 

 two round or oval foramina ; outer surface showing pores, the apertures of short canals 

 which join larger canals radiating from the beak. The thick outer layer of the lower 

 valve is usually brown-coloured and made up of thin horizontal strata, which are in 

 turn composed of small vertical prisms. The white inner layer is porcellanous, and 

 sometimes contains vacant spaces in the lower part of the shell. Three prominent 

 folds are present, on the inner side of the shell, formed by the inbending of both 

 layers of shell, and corresponding to the external grooves {A, B, G). Of these the 

 anterior {A) is longer and thinner than the others, which are thickened at the internal 

 end and carry a small tubercle above. In the two subgenera, Orhignyia Woodward 

 {H. biloculus Lam.), and Batolites Montfort (H. organisans Laj).), the anterior sinus 

 disappears entirely. In Pironaea Menegh., a number of accessory folds appear behind 

 the two columns. According to Douville, the two posterior columns are homologous 



