162 SUPPLEMENT TO THE 



abounds in the Penkill Limestone at Penkill (Upper Llandovery) and at Cuddiston Glen 

 in Ayrshire. Some specimens or fragments of the fossil have also been found by ]\Irs- 

 R. Gray in the Middle Llandovery at Newlands in the Girvan district. It is not positively 

 known to occur lower down, although a fragment of some fossil has been quoted as such 

 in the Explanation of Sheet 15 of the Geological Map of Scotland, and as having been 

 found in the Caradoc of Leadhills. I made every effort to obtain a sight of the specimen, 

 but it had been lost or mislaid, and I hesitate to believe in its existence in the rock or 

 locality alluded to. 



An example of Pentamerus ohlongus was found by Mr. R. Philip in shales forming the 

 uppermost portion of the Upper Llandovery at Minsterley in Shropshire. The Upper- 

 Llandovery Limestone at Norbury in the same county is literally a mass of internal casts 

 and impressions of the exterior of the species under description : many examples measured 

 upwards of 3 inches in length by 2 inches and 8 lines in breadth. 



57. Pentamerus undatus, /. de C. Sow. Dav., Sil. Mon., PI. XIX, figs. 4 — 9; and 



Sil. Sup., PI. IX, figs. 10 to 20. 



Since describing this well-characterised and abundant fossil at p. 155 of my 

 • Silurian Monograph,' I have been able to examine a very large number of better 

 specimens derived from several localities. There appear to exist two well-defined 

 Penfatnerus-zones in the Llandovery rocks of the Girvan district; for, whilst Pentamerus 

 ohlorigus is most abundant in the Upper Llandovery and scarce in the Middle Llandovery, 

 it is in the last-named horizon that Pentamerus undatus is prevalent. The shell is found 

 occasionally with both valves, and they are very convex and even at times gibbous, with 

 the fold in the dorsal valve wide and moderately convex. The sinus in the ventral valve is 

 in some specimens exceedingly broad and shallow with a narrow, median, longitudinal 

 rounded rib ; its margins are bordered likewise by a rounded rib especially observable in 

 many Ayrshire individuals (PL IX, figs. 12, 14). The beak of the ventral valve is often 

 so much incurved as to almost come into contact with the umbo of the dorsal valve, 

 leaving space only for a narrow area. In external shape this shell bears some resemblance 

 to a Spirifera, but it is a chambered and not a spiral-bearing species. The surface of both 

 valves is smooth and marked only by concentric lines of growth. The largest example I 

 have seen measured 16 lines in length by 21 in breadth and 12 in depth. 



Pentamerus undatus is very common in the Middle Llandovery at Woodland Point, in 

 the Girvan district ; but bivalve examples are rare, the fossil being generally found in 

 separate valves. It occurs also at the same horizon in the Newland Beds of the same 

 district, in the condition of internal casts in a light yellow rock. 



