April 26, 1895.] 



SCIENCE. 



453 



beds and under, the name of Conocoryphe 

 irilineata (Atops triliiieatus), is claimed as a 

 characteristic fossil of the Olenellus Zone. 



The genug Microdi-icus is absent. This trilo- 

 bite is especiallj- characteristic of the Olen- 

 ellus Zone and continued to live with Para- 

 doxides. Here it occure in the Paradoxides 

 Zone, but is absent from the Protolenus 

 Fauna. 



The genus Olenellus is absent. Though 

 carefully looked for, no example of this 

 genus has been found among the trilobites 

 of the Protolenus Fauna, hence, though this 

 fauna apparentlj- holds the place where we 

 might naturally expect to find Olenellus, that 

 genus proves to be absent, or at least not at 

 all characteristic ; and, as so many of its as- 

 sociate genera also are absent, we cannot re- 

 gard this fauna as the Fauna of Olenellus. 



Of the genera of trilobites that are pres- 

 ent Micmacca has affinitj' with Zacanthoides. 

 It differs in the course of the posterior ex- 

 terior of the dorsal suture. The relation 

 will seem closer if we suppose a movement 

 of the eyelobe during the growth of Zacan- 

 thoides similar to that which occurred in the 

 Ptychoparidse, by which the eyelobe was 

 drawn in toward the glabella, while at the 

 same time there was a projection of the 

 posterior extension of the dorsal suture out- 

 ward toward the general angle. If this 

 change were shown to have occurred in Za- 

 canthoides, Micmacca might be looked upon as 

 an ancestral form of that genus. 



In this fauna there is a very primitive as- 

 semblage of Brachiopods, of forms which it 

 is in man}- cases difficult to assign to any 

 known genus. Many are small, some are 

 minute, and the larger species belong to the 

 Obolidie and Siphonotretidae. 



The Gasteropoda have already been al- 

 luded to ; among the.se Pelagiella (n. gen.) is 

 remarkable for the peculiar aperture which 

 seems to indicate a free swimming Heter- 

 opod. 



This fauna is distinguished from that of 



Olenellus bj' two marked features; it is 

 more primitive and also more pelagic. 



The way in which the trilobites are bound 

 together bj' the single feature of a continu- 

 ous eyelobe shows a unity of origin and a 

 close relationship not found in any other 

 fauna. And yet among these trilobites 

 there are forms which in other respects are 

 parallel to the types which developed in 

 the later faunas ; thus in Protolenus we have 

 have the flat pleura with the diagonal fur- 

 row of Paradoxides and the deeply grooved, 

 geniculate pleura of Ptychoparia, and at the 

 same time the prominent glabella and deep 

 dorsal furrows of Solenopleura. Micmacca, as 

 has already been said predicated Zacan- 

 thoides of a later fauna, and Protagraxdos in 

 its almost obliterated glabella and flat 

 cephalic shield closely resembles Agraulos 

 of the Paradoxides Fauna. 



It is a more pelagic fauna than that of 

 Olenellus, for we notice the absence of many 

 forms differentiated for shore-conditions. 

 Trilobites with fixed outer cheeks, like 

 Olenellus and Microdiscus are absent ; cal- 

 careous corals and sponges are rare ; thick- 

 shelled brachiopods and the Orthidse are 

 wanting, or rare ; no Lamellibranch is 

 known, but Foraminifera are quite common 

 in some of the beds. 



The question of the antiquity of this fauna 

 as compared with that of Olenellus is dis- 

 cussed. The facies of the fauna as above 

 described indicates a greater antiquity, 

 but if the two faunas were contemporaneous, 

 that of Olenellus may have reached these 

 shores first. 



VOLCANIC DUST IN TEXAS. 

 Sometime since the writer was given, for 

 examination bj' the microscope, a sample of 

 a white, fine-grained silicious deposit by 

 Prof R. T. Hill, of the U. S. Geological 

 Survey, who writes as follows concerning it : 



" Tlie material which I gave you was collected by 

 au old Te.xas friend of miue, Mr. S. P. Ford, in De- 



