312 Mr. G. H. K. Thwaites on a peculiar state of the Palmellse, 



deflected edge; external border convex, much raised and 

 strongly undulato-plicate, the ridges being largest in front, 

 and gradually diminishing towards the external and posterior 

 portion of the tooth ; the plicse are produced by a thickening 

 of the substance of the ridges and a scooping-out of the inter- 

 vening hollows, so that the under side of the tooth remains 

 even ; under surface coarsely osseous ; upper surface polished, 

 with small obscure undulations and minutely porous. 



This genus is closely allied to Ceratodus and Ctenodus, but 

 differs in the grinding surface being concave, the tooth resem- 

 bling the inside of a plicated oyster. The internal microscopic 

 structure was developed for me by the kindness and skilful 

 manipulation of my friend Mr. Anthony of Caius College, Cam- 

 bridge ; it was very complex and peculiar, but the prepared frag- 

 ment has unfortunately been mislaid, so that I am unable now 

 fully to describe it. I only know one species, the following. 



Conchodus ostreaformis (M'Coy). 



About 1J inch long, 1 inch wide, and 1J line thick, the 

 grinding surface deeply concave, the surface of attachment equally 

 convex; the external semicircular margin gives origin to six or 

 seven coarse, rugged, converging ridges, the most anterior about 

 7 lines long and slightly inclined to the straight inner mar- 

 gin, the most posterior is about 2 lines long and at right angles 

 with the inner margin ; the ridges are separated by deep, wide 

 hollows. 



Found along with Dendrodus latus (Ow.), Holop. giganteus 

 (Ag.), and Hoi. princeps (M'Coy), in the old red conglomerate of 

 Scat Craig. 



(Col. University of Cambridge.) 



XXXIII. — On an apparently undescribed state of the Palmellese ; 

 with a few observations on Gemmation in the lower tribes of 

 Plants. By G. H. K. Thwaites, Lecturer on Botany and 

 Vegetable Physiology in the Bristol Medical School. 



[With a Plate.] 



The importance — the necessity, it may be said, — of an acquaint- 

 ance with the lower forms of the vegetable kingdom, in order to 

 afford a clear insight into the real character of the phenomena 

 of growth and reproduction in the higher tribes of plants, is 

 now pretty generally felt and acknowledged by physiologists. 

 From the study of the simpler organisms only can we hope to 

 obtain a correct understanding of the changes which take place 



