24 Mr. J. Lycctt on the Fossil Conchobgy of the 



fissure upon the body whorl, which approaches the outer lip, but does not 

 reach it. This alone is sufficient to distinguish it from Pleurotomaria, 

 from which also the base materially differs, its deep concavity resembling 

 an umbilicus and giving to the aperture a semilunar figure. 



The outer lip is thick, the whorls usually angular and concentrically 

 straited. They occur throughout all the lower fossiliferous beds. Perhaps 

 1 may be excused for briefly alluding to the name given to this shell, 

 although the matter is of a somewhat personal nature. In the autumn of 

 1841, finding that this form was entirely unknown, I forwarded a specimen 

 to Pro. Sedgwick as a new genus, and mentioned that I proposed to call it 

 Trochotoma: about the same time a gentleman who then collected largely 

 from our Great Oolite and distributed its fossils widely always affixed to it 

 the name which I had proposed to give it, so that the appellation became 

 current wherever a collection of our fossils existed six years ago. Within 

 two years afterwards Professor Ansted figured one of the species in his work 

 on Geology under the same name. Knowing these facts, my surprise may be 

 imagined, when lately turning to the new work on Natural History by Pictet, 

 published at Geneva, I found that he had described this genus under a new 

 name, saying that this is the Trochotoma of M. Deslongchamp, and referring 

 to a paper by that gentleman on the Great Oolite of Normandy, published 

 in the 7th volume ofthe*Transactions' of the Linnsean Society of Normandy 

 in 1842. In that memoir are figured and described five species, of which 

 three are found in this vicinity. It would therefore appear that M. 

 Deslongchamp must have read his paper to the Society in 1841, and nearly 

 simultaneously with myself must have imagined the same new word as 

 a designation for a certain new form. The paper in question is even now so 

 little known in this country that I was compelled to resort to the British 

 Museum to see a copy of it. Probably another coincidence exactly similar 

 to this is not upon record. 



It is proposed to restore the forgotten term Cylindrites used by Llywhyd 

 as a gereric name for a form which requires to be distinguished, and which 

 appears to be very characteristic both of this rock and the Inferior Oolite. 

 We possess six species, three of which have been figured, two as Actceon in 

 the " Mineral Conchology," and one as Conus by Archiac ; the generic 

 characters are as follows: — Form cylindrical. Spire small, acute, sometimes 

 not rising above the body whorl but always exposed ; whorls several, 

 usually flat, sulcated at their junctions. Aperture elongated, narrow, 

 almost linear. Columella with two folds at its base, which is slightly 

 turned outwards at that part; base of the apeture 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 Rcemer 

 Placuna jurensis, 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 striae radiate from the apex on 

 every side ; the under surface 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 stria? proper to those shells causing the elevations 

 upon the attached shells. 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 



