34 JOHAN KIvER. [SEC. ARCT. EXP. FRAM 



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

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

 specimen. I have therefore no doubt that the dorsal lobe of the tail 

 was situated on the right side. This is also the dorsal side of 

 the body, as will he observed in the specimen. 



Where then was the cloacal opening situated? In accordance with 

 the views we have stated, it must naturally have been behind the large 

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

 not begin untill some little distance from the rear of this plate, and this 

 is naturally the situation of the opening. It 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. 



We 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, AGASSF/ made 4 species of the genus 

 Psammosteus, all based upon small fragments with somewhat different 

 exterior sculpture. 



Psammosteus maeandrinus, AG., is thus based upon a little frag- 

 ment from Ladoga; the three others, Ps. paradoxus, AG., arenatus, Ac., 

 and undulatus, AG. are based upon small fragments from the Riga 

 district 1 . 



PANDER- who had a large collection of Psammosteus remains, regarded 

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

 ments of one and the same form. He considered them possibly ichtydorulites 

 and fulcra of an otherwise unknown cartilaginous fish, or as having 

 belonged to the caudal part of Asterolepis, and possibly some other 

 varieties of Placoderms. 



1 AGASSI/,, Monog. des. pois. fos. dii vieux gres rouge, 1844, pag. 103. 



2 PANDER. Ueber die Placodermen des dovon. Systems, 1857, pag. 20 and following 

 pages. 



