DAVIDSON — SCOTTISH OAUDONIFFIROUS URACHIOPODA. 
103 
and the name distorta should therefore be retained only as a varietal designa- 
tion. . » . . 
In 1813, Prof, de Koninck observed with justice that it was impossil)lc to 
di.s) inguish t he Silurian Stropli. rhombouhilis from the Carboniferous St. anolof/a, 
and ho united the two, as well as several others, under a single denomination ; 
and although tlio generality of paleontologists have prefcirred retaining the two 
as distinct s]iccics, they have also admitted that it was scarcely possible to 
separate the; Silurian, Devonian, and Carboniferous speeinieus. Botli varieties 
have been found in Scotland; wliilc the typical form has sometimes attained an 
inch and a-quarter in lengtii by two and a-haK in breadth, and still larger indi- 
viduals have been obtained in England and in Ireland. In Stii'lingshirc the 
tyjiical variety is found in the Canipsie main-limestone and Corrie Burn beds ; 
wliilc the variety diMoii.a occurs at Gare in Lanarkshire, at two huncked and 
tliirty-nuic fathoms below the " EU coal," and thi-ee hundred and forty -three at 
Waygatcshaw ; in llenfrewshire, at Davieland-quarry, near Thoruliebank. 
Sub-genus Streptoehynchus. King. 1849. 
The shells composing tliis sub-genus arc closely related to Strophomena ; they 
are usually semicircular, convex, or concavo-convex, and externally striated ; 
tlie ventral valve possessing a prolonged and oftentimes bent or tmsted beak. 
But a single species and some varieties occur in Scottish Cai'boniferous strata. 
XXV. — Streptobhynchus cbenistria. Phillips. PL i., figs. 16-25. 
Spirifera crenistria, Phillips' Geol. of Yorkslure, vol. ii., pi. ix., fig. 6, 18.3G. 
Sinrifera ararJmoidea, S. senilis, and S. radialis of Phillips ; Orthis Kellii, 
0. ci/liiidrica, 0. caduca, 0. granulosa (?), and 0. Bechei, M'Coy ; 0. Sharpei, 
Portlock ; and 0. Portlockiana, Semenow, appear to be either synonyms or 
sun pie varieties of Sir. crenistria of Phillips. 
The shells of which this species is composed are more often compressed and 
semicircular ; the hinge-line straight, and either a little shorter or slightly ex- 
ceeding the width of the shell at the prolonged acute cardinal angles. The 
valves vary considerably in their curves and directions ; in some specimens they 
are both convex or much compressed, and straight throughout, while in other 
examples the dorsal one alone is convex, or fiat to a certain distance, when it 
becomes deflected, and assumes a lesser or greater degree of convexity ; tlie 
ventral valve being in the same shell convex at, and to some distance from, the 
beiik, when it becomes slightly concave towards the margins, following the 
curves of the opposite valve. The area in the dorsal valve is Unear, wlule that 
of the ventral one varies much in breadth and regularity on account of the beak 
being sometimes twisted more to the one than to the other side. The ventral 
area is likewise divided by a triangular fissure, covered by a pseudo-deltidium. 
Exteriorly the valves are ornamented with numerous strong radiating crenu- 
lated striae, of unequal thickness, which increase in number towards the margin, 
while the interspaces are similarly and partially occupied by one or more smaller 
longitudinal striae. In the interior of the dorsal valve a strong hinge-tooth is 
situated on either side at the base of the fissure, supported by small dental or 
rostral plates. The muscular impressions form a saucer-shaped depression, 
partially surrounded by a sbghtly elevated ridge ; the occlusor (A) occupies the 
central portion, and forms two small elongated impressions, separated by a 
slightly elevated mesial ridge, and on either side are the larger scars (R), appa- 
rently composed of two parts, the anterior or central being due to the divarica 
tor, while the other or outer one would be produced by the ventral adjuster? 
In the interior of the dorsal valve, the cardinal process (to which were affixed 
the divaricator muscular fibres) is composed of two testaceous projections. 
