252 P^of. Hughes, On the relation of the appearance [May 5, 



Cystideans are found in this formation, and an addition to 

 the brachiopoda in that most important Cambrian and Silurian 

 genus Orthis. Now let us follow the. fortunes of this genus as the 

 world goes on. 



The genus Orthis is first found a long way from ..the base of 

 the Cambrian, only 4 or 5 species having been yet determined 

 from beds older than the Lower Bala. The species and individual 

 specimens are most numerous in the Bala group, and they begin 

 -to die out in the middle of the Silurian, so that by the time we 

 get to the top of the Ludlow Rocks, we find only some varieties 

 of O. elegantula. The interruption at the base of the May Hill 

 Sandstone, whatever its character or amount, makes less difference 

 in the appearance and disappearance of species of Orthis, than 

 the long time in which there is no marked break from the Har- 

 lech to the Bala Beds, or from the base of the Silurian to the top 

 of the same continuous series. We see that in the time of the 

 Upper Ludlow deposits, the day of the Orthides was past, but 

 they were not quite killed off by the great movements which 

 took place before the deposition of the Carboniferous group. A 

 few are found in the Devonian, and O. resupinata gets well up 

 into the Carboniferous. Then they disappear. In the Bala and 

 May Hill periods species of orthis come in, reach their maximum, 

 :and disappear, often characterizing very limited horizons, e.g. 

 O, protensa, O. hirnantensis, 0. sagittifera, O. spiriferoides, O. in- 

 suiaris, &c. 



Other genera of brachiopods, such as Strophmena, yield very 

 similar results. 



To return to the table of strata. The number o.f new species 

 which have been found in the Lingula Flags as compared with 

 the Menevian has induced pala3ontologists to draw a strong 

 boundary line between the two formations. Perhaps the most 

 important new genera are the trilobites, Olenus and Dikello- 

 cephalus. Now it will be useful here to call attention to the 

 remark of Salter that Conocoryphe, which we have seen was one 

 of the earliest trilobites, was intermediate between the Oleni of 

 the Middle Cambrian and the Calymenid* of the Upper Cam- 

 brian and Silurian. Yet Olenus, of many species, and often very 

 abundant in the Lingula Flags, has not been found below or with 

 Conocoryphe, but is the characteristic fossil of a limited zone, 

 coming in and going out in the midst of continuously and ap- 

 parently somewhat uniformly deposited strata. The brachiopods 

 of the Lingula Flags have had like most of the other fossils dif- 

 ferent specific names assigned to them from those in the under- 

 lying series. 



We must notice the appearance of Niobe in the Lower Tre- 

 madoc preparing us for Ogygia and Asaphus in the Upper 



