Vol. 56.] FOKMAT10N AND ITS GRAPTOLXTB-FAUNA. 453 



posed great palseontological break between them all but disappears. 

 It is true that only one or two species are common to both, but 

 most of the groups of graptolites occurring in the Lower Ludlow 

 Beds are met with in the Wenlock Shales, and the remainder 

 appear to represent the natural and gradual development of allied 

 Wenlock groups. Thus the group of Monograptus dubius, already 

 occurring in the Wenlock, survives undiminished in the Ludlow, 

 and the groups of M. scanicus and M. uncinatus are also represented 

 in both formations. The groups of M. colonus and M. chimcera are 

 probably only developments of that of M, dubius. Even lletiolites 

 is common to both the Wenlock and Ludlow formations. Cyrto- 

 groptus, as such, is unknown in the Lower Ludlow, yet this genus, 

 so valuable for stratigraphical purposes in the Wenlock, may 

 perhaps be regarded rather as a temporary reversion than as a 

 constant biological genus, seeing that in one instance at least — 

 namely that of Cyrtograptus Carruthersi and Monograptus Nilssoni 

 — the two forms appear to be identical in all respects, except in 

 the matter of branching. 



Notes on Terminology, etc. 



The following notes have been drawn up for the purpose of 

 making clear the exact meaning; of the various terms and measure- 

 ments used in the description of the species : — 



1. Owing to the difficulty of observing the exact point of origin of the first 

 theca from the sicula in compressed specimens, its position as here stated is the 

 apparent one only. Thus, in those species which belong to 1 he group of M. dubius, 

 the first theea probably arises above the aperture of the sicula; but its outer 

 wall invariably grows downward, and so appears to arise from the 

 base. 



2. In most species it is found that the number of thecae per inch in the 

 proximal part of the polypary is greater than that in the distal part. Two 

 numbers therefore are given throughout, the first referring to the proximal and 

 the second to the distal end. The first number is obtained by doubling the 

 numbers counted in the proximal half-inch, and similarly with the second. 



3. The length of the theca as given is that of the lower thecal wall, but the 

 amount of overlap is (according to general practice) measured from the upper 

 thecal wall. 



4. The various measurements tabulated in the description of a species are the 

 average of those taken from one or two typical specimens ; thus although the 

 actual numbers may not hold for other specimens, yet the relative proportions 

 are the same for all. 



5. The associates of a species, as here enumerated, are those which have been 

 actually found with it on the same slab. 



(B) Description of the Graptolite-Species. 

 1. Genus Monograptus. 



(a) Group 1. Type M. dubius (Suess). 



1. Thecae of one type only. 



2. Outer wall of first theca extending down to the aperture of 



the sicula and inclined to it at a small angle (20° to 30°). 



3. Length of adult theca, as a rule, 2 or 3 times the width. 



The first two characters are by far the most important for the 



