192 INVERTEBRATE PALAEONTOLOGY 



In the Upper Lias Phyllocems represented its family, 

 Hildoceras was an early Harpoceratid, and Dactylioceras 

 (PI. iv. fig. 3), Peroncceras (PI. iv. fig. 4) and Coeloceras 

 introduced the important Stephanoceratidae. 



Lower Oolitic genera were, in the main, members of 

 the last two families. Lioceras (PI. v. fig. i), Ludwigia, 

 Oppelia and Strigoceras are abundant Bajocian and 

 Bathonian Harpoceratidae, and Stephanoceras, Sphaero- 

 ceras, Morphoceras and Parkinsonia (PI. viii. fig. i) gave 

 similar representation to the Stephanoceratid stock. 

 Macrocephalites was a genus of that family, especially 

 characteristic of the Cornbrash and Callovian. Lytoceras 

 was a long-lived type whose presumed Cretaceous de- 

 scendants showed many points of interest. Ammonites 

 show a marked decrease in abundance in the Upper 

 Jurassic. Cardioceras and Perisphinctes continued the 

 Stephanoceras tradition in Oxfordian and higher horizons 

 respectively; while the Cosmoceratidae (a mainly 

 Cretaceous family) were introduced by Cosmoceras 

 (PL xiv. fig. 7) in the Oxford Clay. Aspidoceras was a 

 form restricted to the Upper Oolites ; it often occurs as 

 loosely-fitting coils of internal moulds of the chambers, 

 giving a " vertebrate" appearance that justifies the appli- 

 cation of the term " snake-stones " to Ammonites. 



In the Lower Cretaceous, gerontic acceleration, 

 coupled with lapse of many previously important types, 

 became abundantly manifest. Olcostephanus was one of 

 the latest of the Stephanoceratidae, Hoplites represented 

 the Cosmoceratidae, and Desmoceras introduced a new 

 family with thickened or constricted apertures. Ancylo- 

 ceras (a Cosmoceratid) showed " uncoiling " characteristic 

 of late evolutional stages. Upper Cretaceous Ammo- 

 noids were the last of their kind. Forms associated 

 with the Lytoceratidae showed the influence of 

 phylogerontic specialization in shell-form. Hamites 



