XXXVl PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



distribution of Trilobites, and its bearings on the view of the arrange- 

 ment and phsenomena of the Silurian formations, as stated above. 



Out of 45 genera of these typically Silurian Crustaceans, 35 are 

 Bohemian, and of the 10 that are not so, 2 (viz. Olenus and Peltura) 

 belong to the primordial fauna, not exclusively however, for Olenus, 

 in our country, ascends higher in the series. Of the second fauna, 

 6 genera are not Bohemian. And out of the entire list 7 genera have 

 been recognized only in Bohemia. Of the species of Trilobites, the 

 number characterizing each of the stages goes on increasing from the 

 primordial fauna to the lower portion of the Upper Silurians, but one 

 species only is common to as many as four of his stages, and, a fact 

 that is worthy of notice, varies in each. The causes of destruction 

 of species are not always clear. M. Barrande attributes due influence 

 to physical changes as regulating their duration, but I must strongly 

 protest against his belief in the doctrine of a limited vitality for each 

 species (" une quantite limitee de force vital"), so that, independent 

 of all other circumstances, each race will necessarily become extinct 

 after a certain lapse of time. I have elsewhere exposed the groundless 

 fallacy of this pernicious hypothesis, — a favourite one with palaeon- 

 tologists, although it can find but few physiologists to give it support. 

 A curious remark is made by M. Barrande, that the species of the 

 more ancient epochs appear to have been more prolific than those of 

 later ages, — a remark doubtless suggested by local phsgnomena. 



When commenting on the general distribution of Trilobites in 

 Palseozoic rocks, M. Barrande calls attention to the fact, that of the 

 44 Silurian genera, three-fourths do not range upwards above the 

 Upper Silurian stages ; 1 1 reach the Devonian epoch, with notable 

 diminution of specific richness, and one only is found in Carboniferous 

 rocks. The generic maximum of Trilobites is concentrated in the 

 Lower, the specific maximum in the Upper Silurians. The direction 

 of the development of the TrUobites is as clearly backwards, so to 

 speak, in time, as that of the Malacostraca is forwards. The same 

 remark may be made on the Brachiopoda as contrasted with the 

 Lamellibranchiata, and the Nautiloida as contrasted with the Ammo- 

 nitoida. On these oppositions I shall have more to say at the 

 termination of this discourse. Most worthy of remark is the fact 

 confirmed by M. Barrande, that the geological position of a species 

 in one region is not necessarily that which it holds in another. This 

 observation is independent of his colonial theory. Thus certain 

 Trilobites are common to the second and third famias in England 

 that are confined to one horizon in Bohemia, and others that are 

 members of the Lower Silurian only in the British islands, are present 

 in the Upper Silurian only in Bohemia. 



An interesting point is the anamorphosis or change of characters 

 within genera in their course through time. M. Barrande's remarks 

 on this matter are highly original and deservmg of study. As 

 instances among Trilobites, I may cite the changes in the course of 

 the grand suture ; the progressive development of the eyes ; the 

 reduction of the thorax ; the increase of the caudal shield ; the change 



