.I011AN KI.KR. [SEC. ARCT. EXP. FRAM 



of the tail, whilst those on the upper side, where the dorsal lohe is 

 seen, are of the -aim- form as that of the right row of fulcra in my 

 -penmen. I have therefore no doubt that the dorsal lohe of the tail 

 was -ituatrd on the right side. Tin- is also the dorsal side of 

 the lmd\ . as will 1 hserved in the specimen. 



Where then was the cloacal opening >ilnated'. J In accordance will) 

 the \iews we have stated, it must naturally have heen behind the large 

 median plate on the under side of the animal. The rows of fulcra do 

 not hegin nntill some little distance from the rear of this plate, and this 

 is naturally the situation of the opening, ft is not surprising that we 

 cannot directly prove this, in the light of the greatly compressed 

 examples of this form. 



General Remarks regarding the two New Species of Psammosteus 



and Other Allied Forms. 



The two new species of Psammosteus which are described in this 

 work, differ distinctly from each other by several easily recognisable 



characters. The histological construction of the cranial skeleton itself 

 provides the best and surest means of identification; but peculiarities of 

 the outer sculpture also appear to be of service. 



\Ve will now see whether these characters can be employed in 

 differentiating the above from remains of Psammosteus and Drepanaspis, 

 which also evidently are closely allied already described. 



AS mentioned before. AGASSI/ made 4 species of the genii- 

 Psammosteus, all based upon small fragments with somewhat different 



exterior sculpture. 



Psammosteus >n<i<'/ui<lrinns. An., is thus based upon a little frag 

 nient from Ladoga: the three others, 7 J .v. imrtHlo.riix. \G.. areiuihts. A<;., 

 and ini<lnl<ilnN, A<;. are based upon small fragments from the Higa 

 district '. 



l'\\ni-:i{- who bad a large collection of /V/wwo.s'/r/fx remains, regarded 

 all of these more or less diffeivntlv ornamented fragments as skeleton ele- 

 ments of one and the same form, lie considered them poillv ichtydomlites 



* 



and fulcra of an olber\\ JM* unknown cartilaginous fish, or as having 

 belonged to the caudal part of J,s7r/-o/r/>/,v, and possibly some oilier 

 ielii-s <.f 



ACASSIZ, MfMii-i:. ilfs. (mis. |',,s. ilii \i.-u\ ^r--s niiifjp, 1M1, |in^. 103. 



I'AM-M: L'l-liiT ilir I'l.-ii-iMl.Tiiii n ,|.'s ili-Miii. Sy-lmis. 1857, pag. 20 and following 

 pag 



