36 JOHAN KI/ER. fsEC. ARCT. EXP. FRAM 



river Aiidoma in the Gouvernement of Olonez, and two plate fragments 

 from the River Aa at Wenden in Livlatid. 



These fragments sliow a similar variation of the outer sculpture as 

 that in the forms I described in this work from Rllesmere Land. 



The spine shaped skeleton element has flattened, crenelated, often 

 markedly extended denticles (Fig. 8 in texl), whilst the fragments of 

 plate which have denticles on one side only, have round, greatly arched 

 denticles, with markedly prominent ribs (PL IV, fig. 1). 



The corresponding parts in the two new forms described here, 

 show similar conditions. Compare PI. V. figs 4—5, and PI. II, fig. 5, PI. 

 Ill, figs 1 — 2, for Ps. ardicus, and PI. V, fig. 3 for Ps. complicatus. 



The ornamentation clearly shows, however, on comparison with the 

 latter a somewhat different type. 



The difference becomes even more apparent when we investigate 

 the microscopic structure. As shown by PI. VI, figs 4 — 6 and fig. S in 

 the text, the denticles have comparatively narrow pulp cavities and in 

 this respect somewhat resemble Ps. complicatus, but however entirely 

 lack the intermediary bundles of dentine tubules which are so charac- 

 teristic of the former. In longitudinal microscopical sections we can 

 often clearly see that (he extended denticles must consist of several 

 fused into one; they show, indeed, several pulp cavities in the direction 

 of their length (Fig. 8 in text). 



This inner structure when compared with the outer sculpture appears 

 to separate the fragments of (he Li\land form that I have had an 

 opportimity of examining quite readily from my new species. The 

 former are most nearly related to Ps. arcticus, which however exhibits 

 a distinct difference as regards the markedly prominent peripheral points 

 of the denticles and their wide pulp cavities. 



In addition to the Baltic Provinces and the region of Lake Ladoga, 

 certain fragments of Psammodeus have been described from Poland 

 (Kielce), the Caithness flagstones at Wick in the north of Scotland, and 

 f|-om Mimer's Valley on Spitzbergen. They were classified by A. 

 Smith-Woodward as agreeing vvilh the forms from the Baltic Provinces. 



DreiMtnaspis Gemllndenensis Schlift., which is geologically older 

 than Psammosteus, as we have shown in the preceding paragraph, is 

 also closely allied. In the structure of its skeleton it greatly I'esembles 

 Psammosteus arcticus, but differs from the same by the somewhat 

 different sculpture and more scattered denticles. There are howe\er, 

 other conditions which cause Drepanaspis to differ from the genuine 

 Psammosteus forms. It is found that several of the caracteristic 



