180 DEVONIAN FAUNA. 



occasionally become paired, especially at the dissepiments. The branches seem to 

 be higher than wide, and the cell-mouths are on their outer surface, on one side or 

 the other of the median row of pillars. There seem to be about three pillars and 

 five cells to the length of a fenestrule. 



Though Phillips gives " Barton " as the only locality, his figured type in the 

 Museum of Practical Geology is from Hope's Nose. Its reticulation is coarser 

 than usual in Lummaton specimens, and its bars may be rather stronger, but I see 

 no reason for supposing it to be distinct. In any case the Barton fossils have a 

 right to Phillips's name. 



The myth that Hemitrypa is a Fenestella clothed with a parasite may be con- 

 sidered utterly exploded, and Phillips's original diagnosis (with the exception of 

 the one inaccuracy, involved unfortunately in the meaning of its name) established. 

 The guard in Hemitrypa is no more parasitic than is the horn of an ox or the tail 

 of a bird. The function, however, of this guard is not very evident. It might be 

 that its purpose was to keep off every substance except food from the delicate 

 tentacles of the animal. But if this were so, it might be asked how it is that both 

 in the Devonian and Carboniferous we find the ordinary unprotected Fenestella 

 dominating. To this question an answer might perhaps have been found in the 

 soft parts of the animals had they been preserved. Or again, its purpose may have 

 been to give strength to the slight expanding funnel which the zoarium forms. 

 Without it an organism of such a shape would be peculiarly liable to destruction, 

 whereas the arrangement of pillars and beams must have rendered it decidedly 

 strong. Judging from the general facies of the Lummaton fauna, it did not inhabit 

 deep water, and was exposed to the action of strong currents and tides, and, if so, 

 the benefit of such supports can be easily understood. 



Affinities. — II. hibemica, M'Coy, 1 differs in its zoarium forming a very much 

 larger, more regular and acute cone, and in its fenestrules being decidedly longer 

 while measuring the same in width. Its cells also seem to be arranged in a more 

 definitely double row. Cole has shown that its reverse face agrees in being striated 

 in the young state, and that the meshes of its guard or tegmen are circular 

 and not hexagonal as described by M'Coy. 



The various American species described by Ulrich are all distinguished by 

 having two distinct ranges of cells in the branches. 



4. Genus. — Isotrypa, Hall, 1885. 



Ulrich thus defines this genus : — " Zoaria infundibuliform. Branches connected 

 by dissepiments. Keel at first very thin, then abruptly expanded ; at intervals, 



1 L844, M'Coy, ' Syn. Carb. Foss. Ireland,' p. 205, pi. xxix, fig. 7 ; and 1893, Cole, 'Sci. Proc. 

 Roy. Dublin Soe.,' vol. viii, n. s., p. 132, pi. viii, n'gs. 1 — 5. 



