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 carine as varieties of S. maximum, but in the 
same Norwich beds, from which Mr. Fitch obtained his fine series of carine, 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 carine, 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 im 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 studymg the cari 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 mstance of so much variation in the scuta; yet 
I believe 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 maximum, with the Varieties I of the scuta and terga, I believe that these valves 
belonged to the same mdividuals: with respect to the two other varieties, I have hardly 
any grounds for conjecturmg 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 Carine, there is a difficulty in 
nomenclature ; for the carina of S. maximum 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 Pollicipes maximus of J. Sowerby, and all the 
other valves, which I have reason to suppose belonged to this species. ‘The other carinze, 
however, as being in this genus the typical valve, are described under separate subordinate 
headings; the description of S. maximum, var. sulcatum, beg given from Mr. Sowerby’s 
original specimen. Under the typical S. maximum, I dicate as far as able, to which 
carinze the varieties of the scuta and terga, there described, probably belonged. 
