SPONOIIDA. 433 



spongologist to whose keen eye lor generic characters we owe this 

 very distinct and constant genus. 



IOTROCHOTA *, g. n. 

 Halichondria, pars, Higgin, Bowerbank, Carter. 



Desmacidinidse with smooth linear skeleton-spicules and minute 

 birotulate flesh-spicules with straight shafts, both the heads being of 

 the same size, circular, and symmetrical ; sarcode purple. 



This genus is formed to include Halichondria birotulata, Higgin 

 (Ann. tV' Mag. Nat. Hist. l s 77, xix. p. 296) and Halichondria pur- 

 purea-, Bowerbank (P. Z. S. L875, p. 293). Halichondria s. str. is 

 based on a Renierid. The peculiar flesh-spicule of this genus is one 

 form of the flesh-spicule which usually appears in the Desmacidi- 

 nidae under the form of an " auchorate," equi- or inequi-anchorate. 

 The latter forms apparently originate by excentric flexion of the 

 shaft of a birotulate form like the present, and suppression of the 

 rays which lie on that side towards which the shaft is bent ; the thin 

 expansions uniting the arms in the birotulate apparently become 

 the " falces " which unite the arms of the anchorate (see Carter, 

 Ann. &llag. Nat. Hist. 1874, xiv. p. 207). An intermediate stage 

 is seen in Ohrondrocladia — viz. 0. virgata, Wyville Thomson, and 

 C. {Halichondria) abyssi, Carter (Vosmaer),— the shaft of the birotu- 

 late beina; bent and the arm of that side almost aborted as in a normal 

 anchorate (see Carter, torn. cit. p. 218). Ohondrocladia differs further 

 from Iotroehota in being accompanied by a bihamate or tricurvate 

 flesh-spicule. Cladorrhiza, Sars (C. abyssicola, id. Some Remark. 

 Forms &c. i. p. Go, pi. vi. figs. 16-34), is an allied form, but not 

 only has the shaft of the birotulate bent, and the symmetry of the 

 head impaired by the almost total reduction of that arm of the head 

 which thus comes into contact with the curve of the shaft, but it is 

 inequi-birotxlate, and corresponds in the birotulate series to the in- 

 equianchorate form of the anchorates of the common types of Desma- 

 cidinid;e ; it differs from Iotroehota in the possession of a bihamate 

 flesh-spicule in addition to the birotulate. 



It is noteworthy that those species of this genus hitherto known 

 are from shallow water (littoral, see below), while all other known 

 allied forms except Axos anchorala, Carter, for which the depth is 

 not given, are from the deep sea. 



From an unusually well-preserved specimen of the green variety 

 of I. purpurea from the Amirante Islands (see Pt. II. of this Report), 

 I am able to make out that the ciliated chambers are oval, the ends 

 being well rounded, and measure *032 by "025 millim. They are 

 crowded along the sides and in the parenchyma, lying between what 

 appear to be secondary and tertiary canals of the excretory system, 

 and also (though this may perhaps be merely apparent) upon the 



* From lov, a violet, and rpa yos, a wheel, in allusion to the purple colour and 

 the birotulate flesh-spicules. 



2f 



