ON SOME PELMATOZOA FROM CHAZY LIMESTONE OF NEW YORK IO3 



in position. These plates complete the aboral surface of the 

 theca. 



Deltoids. Perhaps the most remarkable structures of the oral 

 surface are the great deltoids — large triangular plates, each of 

 which has come to occupy an entire interambulacral area. The 

 superficial resemblance of these plates to "sharks' teeth" evi- 

 dently suggested the specific name, and such plates have induced 

 local collectors to stoutly maintain that " big fish " existed in 

 Chazy time. 



These interambulacral plates have the middle basal portion 

 strongly bent inward and the lateral portions rather markedly 

 outward ; a vertical section of one such plate would thus be con- 

 vex outwardly and a transverse section concave outwardly. 

 The two upper or lateral sides of each plate support on each 

 edge from 6 (or less) to 40 (or more) brachioles with their cor- 

 responding adambulacral or flooring plates. 



Each bordering brachiole (with the possible exception of 

 one of the apical pair) is connected, by a pore or slit betzveen the 

 adambulacrals, with a vertical or rather a longitudinal groove 

 on the inner surface of the deltoid. From both sides of this 

 groove there extends a remarkably thin respiratory sheet, 

 slightly bent away from its fellow soon after its origin and 

 usually still more so near its inner margin where the edges 

 meet in a rounded roof and form a lamellar cavity through 

 which was maintained a How of sea water which made its exit 

 at a short slit at the base of the deltoid. This row of basal slits, 

 one for each hydrospire, is very clearly shown in plate 5 at j. 

 This figure is reduced from a photographic enlargement and as 

 the slits did not appear as dark as in the specimen, the seven 

 outer ones were darkened with india ink ; the remaining slits 

 have been reproduced without retouching of any kind. The 

 bibrachial shown in the figiu'e had been moved slightly inward 

 from its natural position and the specimen thus shows the water 

 exits to better advantage. The arrangement of these hydro- 

 spires on the plate may be seen in figure 2 which is from a 

 camera lucida drawing of a cross-section of the right postero- 

 lateral interradial deltoid made at, or just below the 17th brach- 

 iole and which show^s 34 h3^drospires. 



The most rapid increase of growth of the deltoid was at the 

 two lower angles where new brachioles were regularly added 

 at the side of and just below the last formed, until the number 

 became perhaps as many as 50 on a side, giving a probable total 



