370 Messrs. Alder and Hancock on the Branchial 



and not acuminate, nor so wide at the base as those of B.fontana. 

 The fertile plants from Mr. M'lvor are very different in appearance, 

 being very much smaller ; yet they appear to belong to the same 

 species. 



It seems impossible in the absence of authentic specimens to ascer- 

 tain if this be the B.fontana fi. pumila, " caule abbreviate simpli- 

 ciusculo ; foliis lanceolatis," of Turner, Muse. Hibern. p. 107. t. 10. 

 f. 1 . The only part of his description that points to B. pumila is 

 " foliis intensius viridibus, nee acuminatis, nee cuspidatis," besides 

 the observations that it is much more slender and shorter than a. 

 The figure does not at all resemble the fertile B. pumila from Dollar, 

 and that of the leaf represents it as described, "multo luculentius 

 serratis ; from which it is just possible that the plant intended by 

 Turner may be B. rigida, which has been gathered in Ireland. 



156. B. pomiformis, Hedw. 



On sandy banks : not very common. 



Tribe VII. Mniace^e. 

 Genus 1. Fissidens, Hedw. 



157. F. taxifolius (Linn.), Hedw. 



Very common. 



Stems of this species may sometimes be found with the seta arising 

 from just below the top. 



[To be continued.] 



XXX. — On the Branchial Currents in Pholas and Mya. 

 By Joshua Alder and Albany Hancock. 



[With a Plate.] 



The existence of branchial currents in the Bivalve Mollusca, 

 produced by the action of cilia, and admitted and discharged by 

 different apertures, though denied by one or two authors, may 

 be considered sufficiently established to allow of little further 

 discussion. But though most naturalists admit the existence 

 and action of these currents as a general law, yet exceptions have 

 been claimed for some families and genera, whose anatomical 

 structure is supposed to present an insuperable obstacle to the 

 existence of inhalant and exhalant currents by different siphons ; 

 these siphons, as it is thought, having no communication inter- 

 nally. Among the families so placed are the Myadce and Pho- 

 ladidce. 



Mr. Garner in his excellent essay on the Anatomy of the 

 Lamellibranchiata, published in the ( Transactions of the Zoolo- 



