SCALPELLUM. 27 



its distinctive characters are extremely slight ; but they do not blend away by any inter- 

 mediate forms hitherto seen by me. Looking only thus far, it would have been natural to 

 have classed, without any doubt, all the carinse as varieties of S. maximum, but in the 

 same Norwich beds, from which Mr. Fitch obtained his fine series of carina), there are 

 scuta and terga, which undoubtedly belonged to the genus Scalpellum, and which, from 

 being of equally large size, nearly equally numerous, and having a similar state of surface 

 with the above carinse, I believe belonged to them : but both the terga and scuta 

 present a more remarkable range of variation than do even the carina?. In the case 

 of the terga, at one extreme of the series, I did not even at first recognise the valve to 

 be a tergum ! yet the forms so blend together with very short intervals, that I cannot 

 specifically separate them. Terga of the two extreme forms come, also, from the same 

 localities in Scania. In the case of the scuta there are three distinct forms in Mr. Fitch's 

 collection, which I should certainly have considered as specifically distinct, had I not 

 been led from studying the carinse and terga to believe that this species varies much : 

 moreover, the chief point of variation in the scuta, namely, in the character of the under 

 surface of the upper part, I conceive to be, in some degree, in connection with one chief 

 peculiarity in the terga, namely, the varying prominence of their occludent margins. 

 Although I have not seen any other instance of so much variation in the scuta ; yet 

 I beheve that I have taken the most prudent and correct course in describing them as 

 mere varieties. From the more frequent coincidence of the carina, described as that of the 

 true maximnm, with the Varieties I of the scuta and terga, I believe that these valves 

 belonged to the same individuals : with respect to the two other varieties, I have hardly 

 any grounds for conjecturing which belonged to which. It is most unfortunate that 

 not a single specimen of this species seems, hitherto, to have been found with all, or 

 even a few, of its valves embedded together. 



In giving names to the varieties, as judged of by the Carinae, there is a difficulty in 

 nomenclature ; for the carina of ;S', maximimi and of S. maximum, var. sulcatum, are appa- 

 rently almost equally numerous in the Norwich beds ; and might either be taken as 

 typical of the species ; I have chosen the former name, simply as having been more com- 

 monly used, and from this form having been apparently most widely distributed. I have 

 described under it the original carina of Follici/pes maximus of J. Sowerby, and all the 

 other valves, which I have reason to suppose belonged to this species. The other carinae, 

 however, as being in this genus the typical valve, are described under separate subordinate 

 headings ; the description of S. maximum, var. sulcatum, being given from Mr. Sowerby's 

 original specimen. Under the typical S. maximum, I indicate as far as able, to which 

 carinse the varieties of the scuta and terga, there described, probably belonged. 



