REPORT ON THE TETRACTINELLIDA. Cxxxiii 
owing to its being thus too inclusive that it has not met with general adoption ; as used 
in our scheme of classification the family Pachastrellidse is nearly equivalent to the 
group or subfamily Pachastrellina, Carter, 
The family is closely allied through Pachastrella ahyssi, 0. Schmidt, with the 
Theneidse, its nearest relation in this family being the genus PoeciUastra; by the 
possession of an aphodal chamber-system and sarcenchymatous mesoderm the Pacha- 
strellidse are, however, raised to a higher grade than the Theneidse, and thus in framing 
our phylogeny of the Streptastrosa we have regarded the Theneidse as the ancestral 
group. 
It is only as a matter of convenience, however, that the Pachastrellidse are included 
in the Streptastrosa, since the only genus which possesses the characteristic spiraster is 
Pachastrella itself ; the other two genera of the family difter widely from this, first in the 
absence of rhabdal megascleres, and next in the characters of the microscleres, which in 
Dercitus are a toxa and microrabd, and in Calihropella a spheraster ; the sole character 
by which these genera are united with Pachastrella lies therefore, so far as the spicules 
are concerned, in the calthrops, which is common to all. In Dercitus the characters of the 
chamber-system and mesoderm are not known. 
Genus 1. Pachastrella, 0, Schmidt. 
Pachastrella, O. Schmidt, Spong. Ktiste v. Algier., p. 15, 1868. 
Pachastrellidse in wdiich the megascleres are calthrops and oxeas, the microscleres 
spirasters, microstrongyles, and (?) microxeas. 
Type — Pachastrella monilifer, 0. Schmidt (p. 110). 
Genus 2. Dercitus, Gray. 
Dercitus, Gray, Proc. Zool. Soc. Lond., p. 542, 1867. 
Pachastrella, 0. Schmidt, Spong. Atlant. Gehiet., p. 76, 1870. 
Battershya, Bowerhank, Mon. Brit. Spong., voL iii. p. 347. 
Pachastrellidse in which the microscleres are spined microrabds and toxas. 
Type — Dercitus hucklandi, Bowerbank (p. 108). 
Genus 3. Calihropella, n. gen. 
Pachastrellidse with only one form of microsclere, which is a euaster. The only 
megascleres are calthrops, oxeas being absent. 
Type — Calthropella simplex, n. sp. (p, 107). 
