W. H. Hudleston — On the Yorkshire Oolites. 53 



poda) recognized two species, viz. Monodonta Icevigata, Sow., and 

 Monodonta acmon, D'Orb. 



It is well known that the bulk of these shells from the Gr.O. were 

 classed by Morris and Lycett under Monodonta Labadyei, D'Arch., 

 which belongs to the papilliform section. As far as I can see at 

 present, no particular section can be predicated of any one horizon as 

 being peculiar to it ; but in each district of the Lower Oolites, and 

 possibly higher up in the Jurassic system, some one or more of these 

 sections will be found to prevail, but often with a local modification, 

 which it would be diificult either to describe or depict, but which at 

 the same time is recognizable. Such varieties may be regarded as 

 races. Hence, perhaps, there is more variation laterally than 

 vertically. 



If people please to amuse themselves by giving specific names not 

 only to the sections but also to the races which make up the group of 

 3Ionodonta ItBvigata, well and good ; but such " species " are not of 

 equal value, seeing that they frequently represent differences 

 which are extremely small in amount. 



Description. — var. bellulata, Bean. — Specimen from the Dogger 

 (zone 1), Peak (Blue Wyke). York Museum. Figs. 4, 4a, 4&. 



Height 13'5 millimetres. 



Width 14 ,,• 



Spiral angle 80°. 



Eatio of body-whorl to entire shell 68 : 100. 



Shell small, turbinate, not umbilicated. Whorls four or five, which 

 increase under a regular angle, are moderately tumid, and separated 

 by a well-defined, but not wide, suture. Body-whorl relatively large, 

 tumid, with a very slight flattening of the posterior region. The 

 entire shell is perfectly devoid of ornament, except lines of growth 

 which are numerous and very fine. Aperture tolerably wide and 

 nearly circular; columella thick and strongly toothed anteriorly. 



Relations and Distribution. — This variety belongs to the papilliform 

 section, represented in the Great Oolite by Monodonta Labadyei, 

 D'Arch., and in the Callovian of Montrueil-Bellay by Monodonta 

 papilla, Heb. et Desl. How far these three actually differ from 

 each other it would be difficult to say, without having specimens of 

 each side by side. It is the only section of the group, which is found 

 in any quantity in the Yorkshire Dogger, though by no means 

 common. 



Description of another variety, B. — From the same horizon and 

 locality. York Museum. Figs. 5 and 5a. 



Height 9 millimetres. 



Width 10 



Spiral angle irregular. 



Ratio of body-whorl to entire shell 75 : 100. 



This variety, which is similar to forms from other parts of Eng- 

 land, is characterized by a great irregularity in the development of 

 the whorls. The penultimate exhibits a remarkable bulge anteriorly, 

 whilst the posterior region is considerably flattened. This variety 

 probably occurs wherever the group is abundant. It is very rare 

 in the Dogger. 



