PREFACE. xl 
have given of the creature, however, will be found trie to the 
actual type; and that parts should have been omitted will 
surprise no one who remembers that many hundred belem- 
nites had been figured and described ere a specimen turned 
up in which the horny prolongation, with its enclosed ink-bag, 
was found attached to the calcareous spindle ; and that even 
yet, after many thousand trilobites have been carefully exam- 
ined, it remains a question with the oryctologist, whether this 
crustacean of the earliest periods was furnished with legs, or 
creeped on an abdominal foot, like the snail. 
I owe to the kindness of Mr. Robertson, Inverugie, the 
specimen figured in Plate V., fig. 7, containing shells of the 
only species yet discovered in the Old Red Sandstone of 
Scotland. They occur in the Lower Forrnation of the system, 
in a quarry near Kirkwald, in which the specimen figured, 
with several others of the same kind, was found by Mr. 
Robertson, in the year 1834. In referring to this shell, page 
90, I have spoken of it as a delicate bivalve, much resem- 
bling a Venus; drawing my illustration, naturally enough, 
when describing the shell of an ocean deposit, rather from 
among marine, than fluviatile testacea. I have since submit- 
ted it to Mr. Murchison, who has obligingly written me that 
he * can find no one to say more regarding it than that it is 
however, that they existed as plates of very peculiar form, and greatly 
resembling paddles, and that they served in the economy of the animal 
some still unaccounted for purpose. 
