128 Prof. P. M. Duncan and Mr. W. P. Sladen on 



tubercles parallel to the curved edge. The radial plates 

 are broad adorally and the optic pore is in the adoral edge ; 

 the posterior radials enter the ring of the periproct and are 

 elongate. The radials II., III., and IV. do not enter the peri- 

 proctal ring. A well-defined, small, pentagonal, dorso-central 

 plate is placed in the antero-posterior line of the test and 

 bounds the periproct in front ; its sides are not in contact with 

 basals 1 and 4. A small supplementary plate is on each 

 side of the dorso-central, and these side-plates are not sym- 

 metrical ; they and the dorso-central plate bound the peri- 

 proct anteriorly. 



In examining a large series of specimens of the different 

 species of Acrosalenia it becomes evident that perforate and 

 imperforate tubercles may occur, although the perforate con- 

 dition is by far the commonest. The entrance of a radial plate 

 between the basals, besides the usual radials I. and V. is seen 

 in some specimens of the species even in which the majority 

 of forms have not this character. It follows that the position 

 of the radials, all other characters being the same, is not suffi- 

 cient to alter the specific character, much less the generic. 

 The position of the radials and the perforate or contrary con- 

 dition of the tubercles will be noticed as of no great importance 

 in some types which are about to be described. 



Genus Acrosalenia, Agassiz, 1840, amended. 



Test moderate in size, depressed, tumid at the circular and 

 rarely pentagonal ambitus, rounded above, flatter actinally. 

 Apical system rather large ; four lateral basals large, and the 

 posterior smallest and differing in shape. A dorso-central 

 plate in the antero-posterior axis of the system, in contact 

 with the four larger basals and anterior to the periproct. 

 Supplementary plates to the dorso-central may occur. Poste- 

 rior radials enter the ring of the periproct, rarely a radial sepa- 

 rating the antero- later aland postero-lateral basals more or less. 

 Periproct large and posterior. Ambulacra moderate or narrow, 

 with primary plates near the apical system and with compound 

 plates near the ambitus and actinally. Compound plates of 

 two united primaries with rare demi-plates, except near the 

 peristome. Pairs crowded and biserial near the peristome, from 

 the presence of demi-plates. Tubercles of the interradia 

 largest, perforate and crenulate 5 those of the ambulacra 

 much smaller or only like large granules. Peristome large, 

 decagonal, with well-developed branchial grooves with raised 

 edges. Perignathic girdle with low ridges and slender pro- 



