34 PROCEEDINGS OF TUE 



'I'lie eyes in these (Istnieoderiiis are median as in the Osteo- 

 straci, but the small plate between them, pitted on its inner face 

 for tiie pineal body, is loose, and close to it there is a ))eeuliar 

 complication of jKivts which cainiot yet be explained. Two 

 external plates in front of the mouth nre denticulated, as if they 

 were related to the niar<;in of the mouth itself; and they are 

 notched at the outer Literal ends, as if for nostrils. At the postero- 

 lateral anodes of the head, where tlie i)ranchi:il cliambers seem tO' 

 open in other Ostrju-odeinis, there is a loosely-hinged plate on 

 each side which may be regarded as an oi)erculum. JNlarlungs 

 on the inner face of some of the head-plates si ill await explanation. 



The inner surfaces of the plates of the trunk as a rule exhibit 

 no features specially related to tlie soft parts ; but in one species,. 

 Pterkhthys rhmuinus, from the Middle Devoniaii of the Eifel, 

 Germany, there is a peculiar structure which lias hitherto escaped, 

 general consiileralion. It Mas first noticed by ]iohon * in a 

 specimen which I studied with him in Petrograd in 18'J2, and [ 

 have since observed it in a s-econd s])ecimen in the liritish 

 Museum (no. P. 8882). This structure is a nearly horizontal thin 

 lamina of bone, marked by a longitudinal nu-dian suture^ and 

 thus evidentlv paired, extending lirndy fixed across the hjise of 

 the anterioi- median dor.-al plate anil ending abiuplly just in 

 advance of the posterior border of the plate. In fact, it bounds- 

 below a large dorsal chamber w hich is widely open behind. The 

 nature of this chamber is uncertain, but the vertebral axis must 

 have been well below it ; and it denotes a fundamental difference 

 between the Antiarclii and the Arthrodira (such as Coccosfeits), 

 which are sometimes still regarded as related. 



The tail in Ptenchthi/s is covered with rhombic scales and 

 clearlv heterocercal, with a single sn)all dorsal tin ; but in the 

 other genera it must have been scaleless, and it has only been 

 observed in one species of Boihriolepis peculiarly preserved as a 

 stain on the rock from one formation and locality f. 



The discovery of the tail ot Bothriohpis iji specimens from one 

 very thin layer of rock in a cliff which iiad already been examined 

 by many skilled collectors, illiistrati's well the accidental nature of 

 advances in our knowledge of fossils. AVbole shoals of the same 

 species had been fouiul in other layers of the same section, but 

 without a trace of the tail. In the course of this brief address I 

 have mentioned several cases in which our present acquaintance 

 with important fads depends on one or two la\onrabIy preserved 

 or fortunately broken ^pecimens. 1 lia\e tried to show how a 

 palieontologist correlates tliese isolated facts and arrives at least 

 at a phiusilile conclusion. Our knowledge of all groups of extinct 

 (Mganisms has to be acquired in the same slow and laborious 

 manner. 



* J. V. Rolioii. Vrrliaiull. rus.s. k. min. Ges. St. Petersb. [2] to!, xxviii. 

 (1891), p. 31)8, pi. 7. figs. 1,2. 



■[ \V. I'atien, Tiol. Bulletin, vol. vii. [\904), p. 113, with flgs. 



