THE CARBONIFEROUS OF NORTHUMBERLAND. 749 
Dr. Liitken*, however, has pointed out some time ago the analogy 
of this aperture in Agelacrinitide with the anal tube of recent and 
the so-called “‘ proboscis ” of Paleeozoic Crinoids ; concurrently, there- 
fore, with that view, the opening in question has been spoken of in 
the above description as the periproct or anal aperture. 
Affinities and Differences.—Although the various species of Agela- 
erinitidee present a general facies whose similarity at first sight is 
very striking in a group maintaining such an extended distribution 
both in a vertical and horizontal direction, they will nevertheless be 
found on analysis to exhibit numerous divergences from the type which 
are very difficult to understand, and of which the explanation is most 
hazardous, and, indeed, scarcely possible from the very scanty and 
isolated material which paleeontologists as yet have at their disposal. 
In support of this remark no further instance need be cited than the 
occurrence of the Agelacrinites-form and the Lepidodiscus-form in the 
Lower Silurian, and also both in the Lower Carboniferous as well, 
thus leaving us unable to say which of these should be looked upon 
as the original type; whilst the entire absence of all intermediate 
forms prevents us from throwing any light upon the causes which 
brought about the modification or the stages that have intervened. 
As our knowledge of this interesting group is unfortunately so 
fragmentary, we are unable to do more than point out the external 
characters which distinguish the present form from those with which 
we are already familiar; and a glance at the list which is given 
below will show that the number of species with which we can thus 
draw comparison is very small. 
The Carboniferous forms of Agelacrinitidee have been known to us 
hitherto only from America—the only two species being Agelacrinites 
kaskaskiensis, Hall, and Lepidodiscus squamosus, Meek and Worthen ; 
the former found in the Kaskaskia limestone at Kaskaskia, Illinois, 
and the latter coming from the Keokuk beds of Crawfordsville, 
Indiana. 
The resemblance of Lepidodiscus Lebouri and Agelacrinites kas- 
kaskiensis is merely superticial, for the imbricating plates and the 
character of the radial series furnish diagnostic features which 
readily separate the two species. he affinity with Lepidodiscus 
squamosus is much closer: the Northumberland specimen resembles 
the Indiana form in having imbricating interradial plates; but these 
are distinguished in L. Lebowrt by being much more angular 
(rhombic) in form, and by having their length equal to or greater 
than the breadth, and the margins always rounded; and L. Le- 
bourt further differs in haying larger, and consequently less nume- 
rous, disk-plates, which are also more uniform in size than in 
L. squamosus. There is a wide difference between the two species 
in the structure of the radial series. In ZL, squamosus these consist 
of a double row of narrow plates, but little broader than long, which 
abut against one another in the median line; whilst in LZ, Lebowre 
they are either wedge-shaped pieces alternating with one another, 
or single band-like plates which extend across the ray. It is also 
* Vidensk. Meddel. f. d. Naturh. Forening i Kjébenhayn for 1869, p. 187. 
OrdiG.8.. No. 140. 35 
