180 
THE GEOLOGIST. 
margin ; while again two will sometimes imite, so as to constitute but a single 
rib. The cost SB arc very often of unequal width, rounded, or flattened, and 
sometimes will rapidly iucrease in widtli as they extend towards the margin : 
concentric wrinkles are also observable upon the auriculate portions of the 
valve, and a row of long tubular spines may be seen close to the cardinal edge, 
as well as upon the lateral portions of the beak ; spines of smaller proportions 
l)roject likewise here and there from the ribs themselves, while the longitudinal 
costiE are closely intersected or decussated to some distance from the extremity 
of the beak by numerous undulating concentric lines. The dorsal valve is con- 
cave, but much llattcned to some distance from the hinge-line, while the sculp- 
ture is very similar to that visible upon the opposite valve. No interiors of 
this shell appear to have been hitherto discovered, nor did it ever attain very 
large proportions ; some Scottish examples have measured one iuch and a- 
([uarter in length by about one inch and three-quarters in breadth. 
In his " Monographic du Geme Productus," Prof, de Ivoninck placed 
P. muricatm, Phillips, among the synonyms of the species under description; 
but as the figure in the " Geology of Yorkshire" had given rise to some un- 
certainty, I requested and obtained, through the kind medium of Mr. Dallas, 
the loan of tlie original example, preserved in the Museum at York. It differed, 
however, from the represeni ations that had been given of it in the " Geology of 
Yorkshire," by presenting a well marked median depression in the ventral 
valve, but agreed very closely with certain similar siiells found by Mr. Thom- 
son at Cessnock, iu Ayrshire, and of which fig. 25 of our pi. iv. is an example. 
The presence of so many small tubular spines along the upper surface of all the 
ribs is a character not observable upon the larger number of specimens of P. 
costatus ; and from this character alone I should almost have been inclined to 
separate the last-named shell from P. murkatiis, had not an undoubt ed example 
of the first, in the collection of Dr. Slimon, exhibited a number of similar spines 
along the surface of the ribs. P. confafiis is not a very rare species in Scot- 
land ; it occurs at Hill Head, iu Lanarkshire, at three hundred and seventy- 
five fathoms below " Ell coal ;" also at Brockley, near Lesmahago. In 
Stirlingshire, in the Campsie main limestone. In Dumbartonshire, at Castle- 
cary. In Renfrewsliire, at Barrhead. In Ayrshire, at Iloughwood and V/est 
Broadstone, Beith ; Goldcraig and Monkrcrlding, near Kilwinning ; Auchen- 
skeigh. Dairy ; Meadowfoot, near Drumclog ; and Cessnock, parish of Loudon. 
In Buteshire, in the island of Ai-ran. 
XXXVII. — Productus Youngianus. Dav. PL ii., fig. 26, and pi. v., fig. vii. 
This shell is longitudinally very oval, the hinge-line being rather shorter 
than the width of the sliell. The ventral valve is regidarly arched and without 
any sinus, wliile the auriculate expansions are very small ; and the beak, which 
is comparatively large, does not overlie the hinge-line, except quite at its 
attenuated extremity. The dorsal valve is concave, and follows the curves of 
the opposite one ; exteriorly the surface is ornamented with numerous small 
rounded ribs, of which a certain number are due to intercalation, and from 
which at short distances project slender tubidar spines ; these appearing more 
widely scattered in some s])ecimens than in others. Undulating concentric 
lines of growth are likewise observable, and in some specimens appear to have 
been continued in the form of very tliin and short concentric lameHifcrin ex- 
pansions, but which are on the greater number of specimens broken off close 
to the shell, so that when perfect they nnist have presented a somewhat fringed 
appearance. These lamellif'orm expansions, which lie close to the surface, 
appear to have been more strongly developed or displayed iu the young shell, 
and with age the stria! became more regularly marked, which leads me to sup- 
pose that when quite adidl the shell assumed the appearance of the specimen, 
