254 Mr. J. Lycett on the Fossil Conchology of the 



elongated, narrow, almost linear. Columella with two folds at 

 its base, which is slightly turned outwards at that part ; base of 

 the aperture entire, outer lip thin. All the species are distinct 

 from those of the Inferior Oolite. 



Before quitting this assemblage of shells, another form which 

 has occasioned me much perplexity must be noticed. It is called 

 by Roemer Placunajurensis, but is clearly distinct from that genus, 

 of which it does not possess the cardinal teeth, nor has it the hole 

 or appendage of Anomia. The following are the grounds upon 

 which it is deemed proper to erect it into a distinct genus. 



Generic Character. — Shell very thin, irregular, either convex 

 or flat, posterior border rounded, anterior border more straight ; 

 apex little elevated, but always distinct and placed near to the 

 middle of the anterior border. Fine longitudinal closely arranged 

 waved strise radiate from the apex on every side ; the under sur- 

 face is smooth with a large central impression. 



These shells were frequently (perhaps always) attached to 

 bivalves, more especially to Trigonia, not by the external surface 

 but from the under side ; the knobs and strise proper to those 

 shells causing the elevations upon the attached shell. From 

 these circumstances it would appear that the soft parts of the 

 parasite must have adhered "to the Trigonia prior to the secretion 

 of the thin shelly plate, and that the shelly matter was deposited 

 during such adhesion. On the death of the parasite the thin 

 plate separated, as there was no shelly adhesion between it and 

 the Trigonia, and they are never found attached to the latter. 

 With the scanty knowledge we possess of this form, it would be 

 unwise to speculate upon its affinities unless with great reserve 

 and circumspection. The mode in which the markings of the 

 Trigonice are transferred to this shell renders it very difficult to 

 imagine that it could have been a bivalve. We look as it were 

 upon an impression at the back of the paper, the parts in relief 

 having been stamped through it, but disguised by the finely 

 striated surface at the back of the attached shell. It was seden- 

 tary, and if univalve may have belonged to those forms of the 

 Patelloidea in which the shell is partially enveloped in the soft 

 parts of the animal, examples of which are found in Fissurella, 

 Haliotis, Sigaretus and Stomatia. We would however wish it to 

 be understood that these hints are thrown out chiefly to engage 

 the attention of others, as we are by no means satisfied with the 

 result of our own observations. 



The estuary waters which entombed the varied remains at 

 Stonesfield spread out partially a thin stratum of their muddy 

 deposit to this neighbourhood, without carrying with it any of the 

 forms for which that locality is so famous. Our Stonesfield slate 

 has a few marine shells, among which are Ammonites coronatus 



