36 .HIIIAN Kl.Ki:. [SKI:. AKCT. EXP. FRAM 



river Aiidoma in the Couvernemeiil of olone/. and two plate fragments 

 from tin- Ili\er Aa at Weuden in Livland. 



These fragments show a .similar variation of the outer sculpture a- 

 Ihal in lln- forms I described in this work from Fllesinere Land. 



Tlit- .-pint' shaped skeleton element has flattened, crenelated, otten 



markedly extended denticle- I Fig. S in text), wliilst the fragments of 

 plate \\hich have denticles on one side only, have round, greatly arched 

 denticles, with markedly prominent rihs (|'l. I\'. tig 1 1. 



The corresponding parts in the two new forms described hen 1 , 

 -how >imilar conditions. Compare PI. V, tig- \ 5, and PI. II. i\^. ^. PI. 

 111. figs 1 -2. for Ps. arcticns, and PI. V. fig. o for Ps. coniplicatns. 



Tin 1 ornamentation clearly shows, however, on comparison with the 

 latter a some\\hal 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 

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

 this resprd 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 the extended denticles must consist of several 

 fn-rd into one; they show, indeed, several pulp cavities m the direction 

 of their length (Fig. S in text). 



This inner structure when compared with the outer sculpture appear- 

 to -eparale the fragments of the Livland form that I have had au 

 opportunity of examining quite readily from my new species. The 

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

 a distinct difference a- regards the markedly prominent peripheral points 

 of !h denticles ;,n,l their wide pulp cavities. 



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

 certain fragments of I'xrt in io^lr/<s ha\e been described from Poland 

 Kielce. the Caithness llau-lones al Wick in the north of Scotland, and 

 from Mimer's Valley on Spit/her^en. They were classified by A. 

 SMIIII \\don\\ \ mi as agreeing with the forms Iroin the Baltic Provinces. 

 l)r<'i>,i>isitis (irnniitilcHciisis Scui.i T.. which is geologically older 

 than 1'sinniiKislnts. a- we have shown in the ((receding paragraph, is 

 aUo closely allied. In the >lruclnre of its .skeleton it greatly resembles 

 1'snnuiiustrns nil-Urns, but differs from the same by the somewhat 

 different sculpture and more scattered denticles. There are however. 

 Ilier conditions which cause I ),T/><I >t<i*}>is to differ from the genuine 

 forms. || i> found thai several of the caracteristic 



