ON SOME PELMATOZOA FROM CIIAZY LIMESTONE OF NEW YORK IOO, 



The inner surface of a deltoid is not often seen in as perfect 

 state of preservation as in the material photographed for these 

 plates. The dark calcite, showing the presence of carbon, does 

 not seem to weather so rapidly as pure calcite. This differen- 

 tial weathering has often left the plates in great perfection. 

 The extra deposit of carbon then at the outer closed edge of the 

 hydrospire fold should tend to preserve it, while the less car- 

 bonized filled in ridges between the hydrospire folds and repre- 

 senting the inward thickening of the outer fold would be more 

 rapidly dissolved away and thus leave the more uniform smoother 

 condition usually found. If this idea of the plate extension be 

 correct the stereom of the plate can hardly be said to be folded. 

 The advantage of closing the outer slit of the fold save at its 

 extreme ends will be manifest and the flow secured by connec- 

 tion with the food groove of a brachiole gives us a very effective 

 organ of respiration. Reduced to their lowest terms these 

 hydrospires seem to be diplopores added to a plate through an 

 inward fold of ectoderm and coelomic epithelium at a suture 

 and bridged over across the outer middle of the opening to form 

 a sac with two external openings or pores. In true diplopores 

 there was no extension of the sac and the openings soon became 

 included by the continued growth of the plate around them. In 

 pectinirhombs the fold crossed the suture, a probable extension 

 of the membranes occurred and therefore a separation of the 

 pores through plate growth, the extending lines still indicated 

 at the surface as in Pleurocystis, or obliterated as in Schizocystis. 

 Hydropores of the form of Blastoidocrinus seem to be derived 

 from the diplopore type from plates supporting brachioles and 

 thus coming to be associated with the power to maintain a 

 marked flow of water, in ,which case the neglect to follow the 

 established and inherited rule to close the plate around the pore 

 would be a " weakness " tending to insure survival and the pores 

 would thus come to lie at opposite margins of the plate. 



Brachioles. The brachioles start out at nearly right angles to 

 the edge of the deltoid but at the sixth external plate [the first 

 row shaded or darkened in fig. i] the brachiole assumes a nearly 

 vertical position. The brachioles at the apex are the oldest on 

 the deltoid and are also the longest, being 6 mm long in the 

 specimen figured, and thus as long as one fourth of the extreme 

 hight of the specimen. These oldest brachioles and some six 

 to eight others on each side of them show on their outer sur- 

 faces two rows of alternating plates some 60 in numbe r . 



