158 GEOLOGY AND MINERALOGY. 



UNITY OF PLE0E,AOANTn0S, DIPLODUS, AND SENAQANTHUS. 



Sir Philip Egerton (Ann. Nat. His. jSTo. il9) has ascertained the mutual indentity 

 of the carboniferous fish -types, Plemacanthus* and Diplodus of Agassiz ; and he 

 has also shewn, as previously suspected by Beyrich, that the Xenacanthus Decheni 

 of the Permian beds of Bohemia, &c., is generically identical with Diplodus. Oi'tha- 

 eanthus, Ag., is likewise k closely related form, if not really, as viewed by Gold- 

 fuss, the same type. Considering, says the author, publication as the test of 

 priority, the genera Diplodus (1843) and Xenacanthus (ISiT) must merge into 

 Pleuracanthus, which was put forth in the " Poissons Fossiles " in 1837. 



SILURIAN STAR-FISHES. 



Mr. Salter (Ann. Nat. His. Nov. 185*7) has described a number of new star-fishes 

 — Asteriadse and Ophiuridse — chiefly from a recently discovered locality at 

 Leintwardine in Shropshire, where they occur in the flag-stones of the Lower 

 Ludlow Rock, Species of our New World genus, Palseaster ; Hall — a type 

 ranging in England from the Lower Silurian into the base of the Carboniferous 

 series — are also described in the same paper. The genus is defined by Mr, Salter 

 as follows : — Palceaster, J. Hall. Arms thick, convex, short or moderately elon- 

 gate, and formed of many rows of small spinous ossicles above (with a madre- 

 poric tubercle near the angle of one pair of arms) : ambulacra deep, with trans- 

 verse ossicles, and a single row of large adambulacral plates. Disk plates between 

 the arms, none. 



CIRCULAR POLARIZATION IN CINNABAR. 



M. Descloizeaux (A.nnales de Chimie et de Physique, Nov. 1857) has made the 

 interesting discovery, that Cinnabar, like Quartz, possesses not only a positive 

 axis of double refraetion:>r-coDtrary to the statement of Sir David Brewster — but 

 that it also exhibits circular polarization. As in the case of Quartz also, the latter 

 phenomenon in different specimens is left-handed or right-handed respectively ; 

 whilst in plates cut from twin or interpenetrated crystals, two kinds are simul- 

 taneously iDresent. These facts are the more curious, as no traces of hemihedral 

 modifications have yet been detected in Cinnabar. 



BRACHIOPODA. 



The synoptical view, given below, of the classification groups and principal 

 genera of the Brachiopods, may prove useful to some of our readers. It may be 

 mentioned briefly, that the Brachiopods differ essentially from the Lamellibranchiata, 

 or ordinary acephalous molluscs, by their organs of respiration. The lamellibran- 

 <3hiate mollusc in its adult condition possesses, as breathing organs, two pairs of 

 bi-laminated gills or branehife ; whilst in the brachiopod, respiration is effected by 

 the veined and ciliated lobes of the mantle. The coiled and ciliated arms of the 

 Brachiopoda (from which the order derives its name) appear to be destitute of 

 the large veins of the mantle-lobes, and hence to take no direct part in the respir- 

 atory process. They are supported in many genera by a calcareous loop or special 

 framework attached to the inside of the smaller or entering valve ; but in many 



* A species of Pleuracanthus (P. tuberculatus) lias also been cited by Eichwald from the 

 Devonian rocks of Russia ; and another Devonian form (P. laciniatus) by P. Eoemer, from 

 the Hartz. These species, however, like many others based on fragmentary evidence, should 

 probably be regarded at present as merely nominal.— E, J. C. 



