24 PORIFERA. II. 



/onus that will then be more or less symmetrical. The skeleton consists of polysplenia)- fibres; in the erect 

 forms it is well developed, often dendritically branched, in the massive and incrusting forms it may be of 

 renierid or more irregular structure, or if may in the incrusting ones consist of slightly branched fibres 

 running from the base to the surface without transverse fibres. Spot/gin is most frequently found, in 

 varying, frequently only slight amount. Spicula : Megasclera monactinal, styli or subtylostyli; microsclera; 

 the characteristic microsclera are anisochehv palmatec of one or more than one form, the largest ones 

 often in rosettes: to these may be added sigmata, trichodragmata, toxa. and. rarely, small isochehe palmafte. 



The genus Mycalc, like Esperiopsis, passes through a series of forms from thin incrustations 

 to erect, rather symmetrical forms. The development of the skeleton is connected with this fact. The 

 lowest skeletal structure I take to be the one occurring in some thin, incrusting forms, and consisting 

 of fibres running from the base to the surface and sometimes slightly branching, as has been described 

 and figured by Vosmaer and Pekelharing with regard to M. aegagropila (Verhandl. d. Koninkl. 

 Akad, v. Wetensch. te Amsterdam, 1898, 2, VI, No. 3, PI. I, fig. 3—4). The fibres are here quite free 

 without connecting transverse fibres or transverse spicules. This structure quite without transverse 

 fibres is, perhaps, owing to the fact that these sponges are quite thin crusts. When we get to thicker 

 or massive species, transverse fibres or transverse spicides are found, and the skeleton gets a renierid 

 or more irregular structure which in the erect forms mav pass to a dendritic structure. 



The typical il/ycvz/r-auisochela, as it is found for instance in lingua and placoidcs, is a very 

 characteristic one distinguished by reaching a rather considerable size. It may vary very much in 

 size without showing distinctly marked, different sizes, but it may also occur in several well-marked 

 sizes or forms; the largest ones very often occur in rosettes, which is not known to be the case with 

 regard to the smaller ones. Small isochelse may occur, but rarely, for instance in M. parishii Bow., 

 where they occur together with anisochelse, sigmata, and trichodragmata, and in M. phtmosa Cart., 

 where, according to Carter, they occur together with anisochelae, sigmata, and toxa. Also sigmata 

 may be found of more than one size, for instance in M. macrosigma Liudgren. — The microsclera 

 mentioned in the diagnosis may occur in different combinations. In some species only anisochelae are 

 found, either of one or more than one form, further chelae and sigmata, chelae, sigmata, and tricho- 

 dragmata, which latter combination is very frequent, then chelae, sigmata, and toxa, or chelae and 

 trichodragmata; finally, as mentioned, small isochelse may in a few instances be added to the combination. 



1. M. placoides Cart. 

 PI. IX, Fig. 5 a-1. 



1876. Esperia placoides Carter, Ann. Mag. Nat. Hist. Ser. 4, XVIII, 316, PI. XIII, fig. 12, PL XV, fig. 32. 

 1880. Vosmaer, Notes from the Leyden Museum, II, 147, 32. 



1892. Esperella placoides Topseut, Resultats des Campagn. scient. du Prince de Monaco, Fasc. II, 89, 

 PI. I, fig. 15. 



Erect, somexuhat club-shaped, sometimes compressed or more irregular. Tin dermal membrane 

 thin, without spicules, pierced by projecting spicules, and the surface consequently finely shaggy ; it is 

 provided with sinuous or branched fore-furrows. Oscula in flic upper part of the sponge on the top of 



