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THE AMERICAN NATURALIST. [Vol. XXXVII. 



precisely similar openings occurs in Birkenia on either side of 

 the body at its junction with the head, — that is to say, in a 

 position where they would not be serviceable for the attachment 

 of such organs. These openings are commonly regarded as 

 branchial in function, in which case it is easy to conceive of them 

 as having been diaposed in the two genera relatively as in the 



F 'of' pteridi'thys 1 aspect ° f ' he body armor regard them provisionally as gill- 

 openings, or else declare that their 

 nature is entirely problematical ; Patten's guess we are not pre- 

 pared to take seriously, even as a guess. 



One point further must not escape attention. Our denial of 

 the existence of appendages in Tremataspis is supported not only 

 by analogy and negative evidence, but by the general configura- 

 tion of the head-shield itself. The latter agrees with that of 

 Cephalaspis, Auchenaspis, Drepanaspis and the like, in its convex 

 or regularly elliptical outline, especially as seen from the ventral 

 aspect, where according to Patten the appendages had their 

 attachment (Fig. i). But if we examine the visceral surface 



skate and shark. 1 It is the veri- 

 est flight of fancy to suppose 

 that the series of six lateral open- 

 ings in Birkenia, and nine ventral 

 ones in Tremataspis, imply the 

 existence of as many pairs of seg- 

 mental appendages in the respec- 

 tive forms. And were we to 

 temper our imagination so far as 

 to conceive that only two pairs of 

 incisions served for this purpose, 

 or indeed even one pair, what 

 function should we assign for the 

 rest, since they are all alike ? 

 Given an integral series, why com- 

 plicate matters by postulating a 

 multiple function? Either let us 



