BHITISH UPPER-SILURIAN FENESTELLID^. 243- 



sometimes more together "*. This admirably describes the appearance 

 of the principal incrusting growth found on the base of Fenestella, 

 and is also well shown in several of the drawings ; but it has nothing 

 whatever to do with true Fenestelhl-gro^Yih. There is no doubt about 

 the polyzoary of the Fenestella undergoing a change with age ; but it 

 is not of the erratic character mentioned by Lonsdale, nor by the 

 formation and junction of new cross bars at various angles. All 

 that occurs as the polyzoon attains maturity is that the whole of 

 the existing structure receives an additional thickening of calcareous 

 matter ; and, as will be readily seen, the oldest portion, namely the 

 base, would receive in time the greatest amount of deposit, and con- 

 sequent thickening of its part over the other portions of ihe poly- 

 zoary. Continuing my investigations respecting these incrusting 

 organisms, I have found, in all, four species which thus disfigure and 

 obscure the base of Fenestella. Two of these arc found on Lons- 

 dale's species ; and two I have since discovered. The most common 

 incrustation is apparently a coral, a portion of the case of which 

 fills the hollow spaces between the lines of the interstices, which are 

 crossed in an irregular manner by tabulae, which seem to take the 

 place of the dissepiments of the Fenestella. This particular organism 

 is depicted in four of Lonsdale's drawings of Fenestella prisca t. 

 Another parasite, in the sense I have mentioned, is an Aulopora, 

 whose lines of growth run smoothly along the interstice, greatly in- 

 creasing its dimensions, while here and there are given ofiP the cha- 

 racteristic tubular bodies. This is also seen in the drawing of 

 Fenestella Millerit. Another of the organisms might be an incrust- 

 ing polyzoon, judging from the peculiar branched or lobed bodies 

 which are alternately given off from the stem, which follows the 

 line of the keel of the interstice of the Fenestella. Another species 

 is very similar in structure to some of the Alveolites of the Carboni- 

 ferous Limestone. This- 1 have found incrusting one of the pre- 

 ceding growths. 



It is obvious that in the weathering which the remains have under- 

 gone the outer organism would suffer the most ; notwithstanding 

 this, all the incrusting forms mentioned can be satisfactorily made 

 out. I may mention, in connexion with these incrustations, that the 

 growth invariably commenced at the base, and rarely extended be- 

 yond one inch from that point. I have in no case found it on any other 

 portion of the polyzoarium. Erom this I infer that in time there was 

 a loss of vitality at the base of the polyzoon, which allowed the growth 

 of the several incrustations. 



When Fenestellce became thus obscured by parasites and worn away 

 by weathering, we cease to wonder at the confusion and uncertainty 

 attending Lonsdale's species. And more, with every wish to do 

 justice to an early worker and careful observer among the Polyzoa, 

 it is not possible, with the material left by him at our disposal, to 

 make out the species intended. The only course which I see open is 

 to follow Prof. M'Coy's example, and begin de novo by describing 



* Murch. Sii. Syst. pp. 677, 678. t Murch. Sil. Syst. pi. 15. fig. 15. 



X Murch. Sil. Syst. pi. 15. fig. 17. 



