ALBIAN FOSSILS DISCOVERED AT OKEFORD FITZPAINE. 69 



Ammonites mammillatus, Schlotheiin : Leonhard's, Taschenbuch 

 Mineralogie, 1813, Vol. 7, p. 111. 



Ammonites monile, J. Sowerby : Mineral Conchology, 1816, 

 Vol. 2, pi. 117, p. 35. 



Ammonites clavatus, (De Luc, ms.) Brongniart : Cuvier's 

 Ossemens Fossiles 1822, new edition, Vol. 2, part 2, PI. 6, 

 fig. 14, p.p. 335, 609. 



Ammonites tuberculifera, Lamarck : Hist. Nat. Anirn. sans. 

 Vert., 1822, Vol. 7, p. 639. 



Ammonites mammillaris, Orbigny : Paleontologie Franchise, 

 Terrains Cretaces, Cephalopodes, 1840, Pis. 72-73, p. 249. 



Acanthoceras mammillatum, Neumayr : Zeitscli. deutsch, geol. 

 Ges., 1875, Vol. 27, p. 249. 



Only three fragments of this Cephalopod were found at Okeford 

 Fitzpaine, but as they possess the typical ornamentation of the 

 species there is no doubt as to their accurate determination. Age 

 and probably sexual conditions have produced certain varieties of 

 sculpturing in the shell, so that young specimens are covered with 

 a close moniliform ribbing, whereas older examples have the costs; 

 composed of robust tubercles arranged at distant intervals on the 

 test ; and in the aged state of the shell the tubercles become quite 

 obsolete. Two of the specimens in question illustrate the inter- 

 mediate condition, having widely distant tubercles forming wedge- 

 shaped, tall, and conical protuberances, being divided on the 

 peripheral or ventral area by a median groove encircling the 

 whorls. The wide space between successive tubercles is occupied 

 by two prominent lines, representing the position of tubercles at a 

 younger period of growth. Every tubercle is marked by five or six 

 equi-distant, transverse, and semi-circular ridges, having a smooth 

 concave surface between each. As they extend down the sides of 

 the whorls the tubercles diminish to a point or node, from which a 

 short rib proceeds to the umbilical margin. In the third or more 

 aged specimen the tubercles and peripheral groove are obsolete, the 

 ribs being closer together and extending across without interrup- 

 tion ; only a few obscure nodes are seen on the sides of the whorls. 



