LOWER C'RETACKOUS FOSSILS — KTII KKI l)i iK. 325 



(rciins PaciivdoMKLLA,''' ifii. nor. 



Obs. — The single species comprised in this genus, although 

 diminutive, is a most important one geologically. Within the 

 area from which Mr. Bloomfield's collection was made, it evidently 

 marks a well defined horizon, and occurs in countless numbers. 

 Had it not been for this, I would not have ventured to describe 

 it from the very simple fact that I am unable to give any definite 

 generic characters except the edentulous condition. The slab 

 figured on PI. lix., will afford some idea of the enormous 

 numbers in which this little shell occurs. Furthermore, its 

 external cliaracters are such that it can be easily recognized in 

 the field, and this with its highly gregarious nature will always 

 afford aid to the field geologist. 



Notwithstanding its plentitude and in a comparative sense its 

 thick test, I have been unable to observe either the adductor 

 scars or pallial line ; for all I can see to the contrary the latter is 

 entire. Under these circumstances the following description 

 must serve both as a generic and specific definition. The name 

 must be regarded for the present, simply as one of convenience. 



Paciivdomklla ciiutus"', sp. itur. 



(Plate ixii., figs. 4-S) 



Sp. Char. — Shell small, transversely ovate, trigonal, very slightly 

 inequilateral ; test thick. Valves convex, the convexity increased 

 by one or more epiostraca, the umbonal one usually large and pro- 

 jecting, when more than one on each valve, they overlap from 

 above downwards. Cardinal margins slightly angular ; neither 

 lunule nor escutcheon; articulus edentulous ; umbos prosogyrate, 

 small. Anterior ends slightly less than the posterior, the margins 

 of the former rounded, those of the latter more obtuse ; posterior 

 slope presents, but ill defined. Ventral margins widely semi- 

 circular. Sculpture concentric and delicate. 



Obs — I am not able to suggest even an alliance for this gre- 

 garious mollusc. The name Fachi/domella is not to be taken as 

 thereby indicating a relation to the Permo-Carboniferous genus 

 Pachydomns, it is given simply in allusion to the thick test and 

 small size, and the specific name similarly refers to the prominent 

 overlapping epiostraca, that form so marked a feature on each 



16 7rd;(vs — tliick, So/iaos— liouse. 

 IB ;(UTOS — heaped up. 



