SLIMONIA ACUMINATA. 119 



One other point which may be mentioned as favouring the conclusion we have arrived 

 at, namely, that there were at least two thoracic plates, the one attached to the first thoracic 

 segment coalesced with the cephalon, and bearing the reproductive organs ; the other 

 attached to the second (first free) thoracic somite and bearing the branchiae 1 — is that 

 some of these thoracic plates are closely covered with fine squamate markings, whilst 

 others are entirely destitute of any kind of ornamentation upon their surface. 



I do not think that this is due merely to the condition of preservation of the individual 

 specimen ; for if markings were present we should, from the fine-grained nature of the 

 matrix, certainly find them either upon the intaglio or relievo side of the slabs, 

 the specimens being nearly always obtained thus impressed, in duplicate as it were, in 

 the matrix. 



On the contrary, I am inclined to consider the presence of scale-markings in the one 

 plate and their absence in the other to be due to the circumstance that the scale-marked 

 plate is the outer or first thoracic plate (bearing the ovaries), whilst the smooth plate is 

 the inner or second thoracic plate (the gill-bearing plate), which, like the inner plates to 

 Limulus is thinner and more membranaceous than the external operculum. 



Judging from the large series of detached portions of this species, preserved in the 

 British Museum, and in the Museum of Practical Geology, Jermyn Street, the largest 

 specimen of Slimonia acuminata known could not have exceeded four feet in length ; and 

 by far the greater number of individuals were less than three feet long. They cannot 

 therefore be said to equal in size the largest species of the genus Pterygotus, which, no 

 doubt, attained a length of at least five feet. 



Besides the almost perfect specimen, figured on Plate XVII, which is about 26-| inches 

 in length, there are three small and nearly entire examples of Slimonia, measuring respec- 

 tively : 1\ inches, 7 inches, and 5f inches in length, and \\ inch, \\ inch, and 1 inch 

 in breadth. 



The hinder border of the head and each of the six anterior segments is marked in these 

 young individuals with two small subcentral keels or ridges upon their dorsal surfaces. 



The integument appears to have been extremely thin; no scale-markings are discernible, 

 and the whole surface is much crumpled up. 



Except that at this early stage the young of Slimonia seem to be much narrower in 

 proportion to their length, the restored figures on Plate XX might serve to represent one 

 of these small specimens. Portions preserved in section in the British Museum, and in 

 the Museum of Practical Geology, indicate that the anterior segments in Slimonia were 

 even more robust than in Pterygotus (see Part I, p. 41), whilst the posterior segments 

 were nearly if not quite cylindrical. 



Only one species of this singular genus is known, and its remains are, at present, con- 

 fined to one locality, namely, the Banks of Logan Water, near Lesmahagow, Lanarkshire. 



1 It is quite possible there may have been a third plate, also branchigerous. 



17 



