1879.] of life forms to hreaJcs of continuity in the strata. 255 



plentifully in the Bala Beds. But it was in the Wenlock of all 

 the older rocks that the tribe of stone lilies flourished . most, 

 while the starfishes are most developed in the Ludlow, Encrinites 

 appeared abundantly now and then in later times, often cha- 

 racterizing deposits of small extent horizontally and vertically; 

 Woodocrinus, for instance, having been found only at Richmond in 

 Yorkshire, and E. liliiformis being confined to the IMuschelkalk. 

 Representative forms exist at the present day. Changes of cur- 

 rents, of temperature, and of their floating food, &c. must have 

 caused them to disappear from the areas where they were once so 

 abundant, for though Encrinites were flxed they did not grow like 

 a plant, and got no more nourishment from the soil where they 

 flourished than an oyster from' the outside of an old bottle on 

 which we find them sometimes growing. Encrinites could not 

 migrate, but as in the case of oysters their spat might. 



Though several genera of Gasteropoda occur at various 

 horizons in the Bala Beds, they are never sufliciently numerous 

 to allow us to infer anything from their absence elsewhere. 



I have followed Orthoceras as the earliest Cephalopod through 

 its range, but if we take any other Cambrian or Silurian form, and 

 notably Lituites, we shall see how common forms begin and end 

 in the middle of uninterrupted deposits. Phragmoceras certainly 

 begins with the Silurian, but seems to be the only genus of the 

 Cephalopoda that does so. 



Quite at the base of the Silurian (i. e. in the May Hill Sandstone 

 including Lower and Upper Llandovery) a number of. new forms 

 are seen for the first time. All that marked group of brachio- 

 podous shells, the Pentameri, and with them Stricklandinia, are 

 here first strongly represented. They have not all been found down 

 to the base of the group. Stricklandinia lens appears early. Pen- 

 tamerus oblongus and P. globosus hardly occur below the upper 

 division. But though they come in suddenly after a break they go 

 out suddenly in the midst of continuous deposits, before we 

 get fairly into the Wenlock. Another species, the Pentamerus 

 Knightii, a large and well-marked form just appears in the middle 

 of the Ludlow Rocks, being almost confined to the Aymestry Lime- 

 stone. Of Meristella we may tell the same tale (just expressing a 

 doubt as to Meristella angustifrons). It is a genus very characteristic 

 of the base of the Silurian, dying out by species as we ascend. 

 The genera Leptaena and Strophomena have a long range,but rarely 

 either generally or specifically are their appearances coincident 

 with any physical breaks. 



I will not dwell much more on the details of the Silurian 

 fossils, many of which I have already commented upon in tracing 

 types up from the Cambrian, but I may remark in passing 

 that the evidence we obtain from them is just the same. 



