(54 



his ' History of British Animals, 1 it must be referred to the genus 

 Halickondria, and I have given it the specific name of Johnstonia, 

 in honor of Dr. Johnston of Berwick-on-Tweed, whose ' History ot 

 British Zoophytes ' is so well and so favorably known to the public, 

 and who has a ' History of British Sponges ' nearly ready for publi- 

 cation. 



Plate VI. fig. 1. 



Halickondria* Johnstonia. — Sessile, massive, surface undulating. 

 Terminations of the excurrent canals disposed on the most elevated 

 parts of the surface, depressed, sometimes bounded by a slightly ele- 

 vated marginal ring. Incurrent canals inconspicuous. Colour of 

 the crust slate-grey, of the interior substance yellow. Spicula of the 

 skeleton mostly simple and slightly curved, rarely attenuating towards 

 their ends, but usually of equal diameter throughout their whole 

 length, having hemispherical terminations ; sometimes, but rarely, tri- 

 radiate or multi-radiate. Spicula of the flesh minute, stellate ; rays 

 attenuating regularly to their apices. Spicula of the crust minute, 

 simple and fusiform, having their surfaces irregularly tuberculated, 

 and their terminations very obtuse. 



This sponge is found attached to the Thatcher and other rocks, in 

 the neighbourhood of Torquay, about low water-mark. The parts of 

 the crust which are most exposed are of a dark grey colour, sometimes 

 almost approaching to black ; while the marginal portion near the 

 base is frequently of a light grey, and occasionally almost white. 

 The crust varies from about a quarter to half a line in thickness. 

 The interior substance is of a dull yellow colour, firm in texture, and 

 very much resembling the crumb of bread in appearance. It is tra- 

 versed by numerous large excurrent canals, of about the tenth of an 

 inch in diameter, which pass in a tortuous direction from the base of 

 the sponge to the most elevated parts of its surface, where they ter- 



* Since this paper was read at the Microscopical Society, I have consulted with 

 my friend Dr. Johnston of Berwick-on-Tweed, upon the propriety of making this 

 species the type of a new genus ; and from the well-marked difference in structure 

 that exists hetween it and and the most characteristic species of Halivhondria , it has 

 heen thought advisable that it should be separated from the latter. I have therefore 

 named it Pachymatisma, from 7ra%yj, densus, and J/xaTitr/xoj, vestitus, and tbe follow- 

 ing are the characters adopted : — 



Spongia carnosa, Crustacea, nee cellulosa nee vi resiliendi praedita, cute crassaJaBvi 

 osculis sparsis perforatis obsita ; spiculis siliceis ct aciculiformis et stellatis sparsis et 

 gemmulis siliceis ubique commixtis referta. 



