303 PROCEEDINGS OF SOCIETIES. 



Structure of Gill on Centrostephanus Bodgersii. — Mr. Mackin- 

 tosh exhibited one of the external gills of Centrostephanus 

 Rodgersii (A. Agassiz). It consists of an elongate axis or rachis, 

 along which are tufts of membrane, arranged in pairs, to nearly 

 the distal end of the gill, where they become confluent in the 

 middle line and continue as a ridge to the termination. The axis 

 of the gill is supported by a series of calcareous rods with their 

 ends overlapping each otiier, and scattered through the rest of 

 the organ are numerous spicules varying from simple rods to 

 large perforated plates, very like but distinguishable from those 

 of Strongylocentrotus lividus figured by Valentine in his * Ana- 

 tomie du genre Echinus,' (in Agassiz's ' Monographies des Echi- 

 nodermes'). An external gill of Echinus esculentuSjlAnn^via,^ 

 also shown for the sake of contrast. 



New species of Chytridium, parasitic on JEctocarpus pusillus. — 

 Dr. E. Perceval Wright exhibited specimens of a new species of 

 Chytridium parasitic in the cells of Metacarpus pusillus. On 

 some of the twigs of the Ectocarpus almost every second cell 

 contained a specimen of tlie parasite, which often caused the 

 growth of the plant to become pushed into an irregular and 

 knobbed form. When first observed the circular Chytridian cells 

 were regarded as oval '' fruits " of the Ectocarpus, but on the 

 development being watched side by side with that of the tricho- 

 eporangia with which the Ectocarpus was crowded, the difference 

 in the spores was shortly quite apparent— those of the Chytri- 

 dium being uniciliated, and the nucleus of each spore the first 

 indication of spore-formation ; whilst those of the trichosporan 

 gium were as described by Thuret and quite of an algal character 

 Specimens of Ectocarpus crinitus, Harvey, were shown in fruit 

 as described by Harvey, which fruit was, without doubt, the same 

 species of Chytridium as now described. Specimens oi Ectocarpus 

 Mertensii were also shown to prove that the so-called " utricular 

 fruits" were really only modified trichosporaugia, and it was 

 suggested that possibly the oosporangia of Thuret might, in some 

 cases, be either modified trichosporaugia or parasitic Chytridae. 



15th February, 1877. 



Microscopic Structure of Noble Opal. — Professor E. Hull, 

 E.R.S., exhibited under the microscope a thin section of noble 

 opal from Australia. The specimens of opal in this instance were 

 contained in a sort of tufaceous material, of a brownish colour, 

 highly vesicular, not calcareous, and of unknown origin. The 

 opals were exceedingly numerous in the paste, of various sizes up 

 to a quarter of an inch, and of varying characters, from semi-opal 

 to rich fiery or noble opal. The smaller specimens were the most 

 opalescent. 



The slice under a high power exhibited some minute cells at 

 intervals, but these were apparently unconnected with the pro- 

 perty of opalescence. In the portion of the specimen more in- 



