pe 
1888.) 85 [ Williams. 
radial plates. The arm plates pretty generally have a small tubercle for 
each plate, but there is an occasional exception. 
From these tubercles proceed slender, acicular spines, bristling outward 
from the calyx and arms, and upward from the vault. 
These spines were evidently movable, and articulated by? ligaments or 
?muscle upon the pitted tubercles. The spines are also pitted at their 
bases, 
The typical species, A. Jthacensis, is from the base of the Chemung 
group at Ithaca, N. Y., from a fine, sandy shale, containing Spirifera me- 
socostalis, Productella speciosa, Strophodonta mucronata, and other Che- 
mung fossils, and the specimens are in the museum of Cornell University. 
Arthroacantha Ithacensis, n. 8. 
This name is proposed for the typical species upon which the genus 
Arthroacantha is founded, and the imperfection of the material and the 
actual variation among the few specimens seen create considerable doubt 
4s to what may be the permanent characters which distinguish the typical 
form from other representatives of the genus, I will give therefore a par- 
ticular account of the size and proportions of the parts of the typical speci- 
mens, and remark upon the variations observed. 
The general shape and features are described in the generic diagnosis. 
The typical species has the following dimensions : 
MM. 
DeLee NGS Wale Viuie eb rslahe eisai Ha WIEN Vee Geleleieieie BNO 
DHSHCUL ee ned ON bay Kae em bases we bewa er MOCO! 
Aviing, (tHiclmesBvat WASS i UAN Wece Meee ull Niall Td 
estimated length ..... EO iC OVO 
Stem, thickness at junction with calyx........... 1.9 
Basal, radius from center of disc to base of Ist \ 
re 
PROTA erceeivis swwts PRE a gi Giase pS NAL Wyarued 
SU ECLA ps ALOT vss digs perdiviais' viv esi eal vimicaperNiatale epic (MeO 
nt 
Bd BHA, SA radials tOMSvHON iis sinrivia eore vieyrlaia sane t 
Lat TACIA AWAGUELU DASOhis Uateleeiiels eciea-win Vise hiNiwers ru O80 
Fe ices UD Muse wre TaMMiRlure KuMbendTy N's. Gree V aoe Chua! 
MUDELCLES, Ciatneter MW Uwe Wit wwiaewitiy 6 we eyed sui 0.6 
number on one basal plate....... 2006 16-18 
"s FO OMA et CACHAN Grell Ww hes evecw 21 
DPMS LENSE OF LOMBCK su Wdilew 6 Vibesirs viele ayes eee | GaeO 
TAMVEUGU HL, ELSIE Wi tie wuethiw chu volwicicere’n Vavouwcanaib, ui eee 
The arms have ten to fifteen joints before they bifurcate. The calyx 
plates are marked on the inside by several distinct lines parallel with each 
other and with the outline of:the plate, arranged concentrically like lines 
of growth. This feature is not seen on the specimens from the Hamilton 
group. 
There is considerable difference in size among the several specimens 
from the typical locality, though the majority of specimens are about the 
