86 
C^ASTT : FOSSIL MOLLUSC A. 
Succinea, Limax, Bulimus, Zonites ( = Ilelicella), Helix, Cochli- 
eopa ( = Zua), Clausilia, Achatina, Pupa, Vertigo, Carychium, 
Acme. 
Lamellibranchiata, — Pholas, Saxicava, Mya, Corbula, 
Thracia, Mactra, Tellina, Donax, Scrobicularia, Venus, Cyprina, 
Sphoerium, ( = Cyclas), Cyrena, Pisidium, Cardita, Cardium, 
» Unio, Anadonta, Astarte, Montacuta, Mytilus, Modiolaj 
Pectunculus, Nucula, Leda, Anomia. 
From the foregoing lists we may form an idea of the 
comparative richness of the different formations of Yorkshire in 
fossil mollusjca. It is my intention to present to the readers of 
the Yorkshire Geological and Polytechnic Society, from time to 
time, sketches of this molluscan fauna, and to take up the subject 
mainly in its Palasontological aspects. The first group proposed 
to be dealt with is the Cephalopoda, but before proceeding to 
any description of species we present the modern classification of 
the gTOup as proposed by "Dr. Paul Fischer in the Journal de 
Conchyliologie 3'' Serie, Tome xxii. No. 1. Paris, 1882, where 
he says, " For a long time the Class of the Cephalopoda has jbeen 
subdivided into two orders. 1st. The Dibranchiata or Acetab- 
ulifera. 2nd. The Tetrabranchiata or Tentaculifera. I think it 
is now necessary to adopt a third order, that of the Ammonea in 
which the characteristics are mixed, approaching to the 
Dibranchiata by the initial sac of the shell in which is found a 
siphonal coecum whose extremity does not touch the walls of the 
sac, but differing from the Dibranchiata by the animal having 
been always protected by an external shell to which it adheres 
by the aid of an adductor muscle; and by the absence of an ink- 
bag. On the other hand, if the Ammonea resemble the Tetra- 
branchiata by their external shell, and the absence of an ink-bag, 
they are easily distinguished from them by their initial shell 
without scar, and by the existence of one or two solid pieces 
(Aptychus) found in the chamber of habitation. The order 
Ammonea ought therefore to be intercalated betwixt the 
