180 



THE GEOLOGIST. 



margin ; while again two will sometimes unite, so as to constitute but a single 

 rib. The costse are very often of unequal width, rounded, or flattened, and 

 sometimes will rapidly increase in width 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 

 project likewise here and there from the ribs themselves, while the longitudinal 

 costse 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 flattened 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 inch and a- 

 quarter in length by about one inch and three-quarters in breadth. 



In his " Monographic du Geme Productus," Prof, de Koninck placed 

 P. miiricatus, 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 the original example, preserved in the Museum at York. It differed, 

 however, from the representations that had been given of it in the " Geology of 

 Yorkshire," by presentmg a well marked median depression in the ventral 

 valve, but agreed very closely with certain similar shells found by Mr. Thom- 

 son at Cessnock, in Ayrshire, and of which flg. 25 of our pi. iv. is an example. 

 The presence of so many small tubular spines along the upper surface of aU the 

 ribs is a character not observable upon the larger number of specimens of P. 

 costahis ; and from this character alone I should almost have been inclined to 

 separate the last-named shell from P. muricatus, had not an undoubted example 

 of the first, in the collection of Dr. Slimon, exhibited a number of similar spines 

 along the surface of the ribs. P. cosfatus is not a very rare species in Scot- 

 laud ; it occurs at Hill Head, in Lanarkshire, at three hundred and seventy- 

 five fathoms below " Ell coal ;" also at Brockley, near Lesmahago. In 

 Stii'lingbhire, in the Campsie main limestone. In Dumbartonshire, at Castle- 

 cary. In Renfrewshire, at Barrhead. In Ayrshire, at Roughwood and West 

 Broadstone, Beith ; Goldcraig and Monkreclding, near Kilwiuning; Auchen- 

 skeigh. Dairy ; Meadowfoot, near Drumclog ; and Cessnock, parish of Loudon, 

 In Buteshire, in the island of Arran. 



XXXVIL — Productus Youngianus. Dav. PI. ii., fig. 26, and pl.iv., fig. vii. 



This shell is longitudinally very oval, the hinge-line being rather shorter 

 than the width of the shell. The ventral valve is regularly arched and without 

 any sinus, while the auriculate expansions are very sm;ill; 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 

 whicli at short distances project slender tubular spines ; these appearing more 

 widely scattered in some specimens 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 thin and short concentric lamelliform ex- 

 pansions, but which are on the greater number of specimens broken off close 

 to the shell, so that when perfect they must have presented a somewhat fringed 

 appearance. These lamelliform expansions, which lie close to the sm-face, 

 Jippear to have been more strongly developed or disj)laycd in the young shell, 

 and with age tlic stria^ became more regularly marked, which leads me to sup- 

 ]K)se that when quite adidt Ihe shell assumed the appearance of the sj)ccinien. 



