of the Ventriculidse of the Chalk. 211 



cuncialis is smooth. It will be perceived that where, as in fig. 12, 

 the folds are broad on each surface at their margin and narrow 

 to a point at their base, the effect must be to give to the whole 

 wall of the pouch a rough surface. The wall is usually not very 

 thick. I have never seen it attain so great a thickness as V. 

 quincuncialis often does. The height and depth of the folds 

 render it difficult ever to confound it with V. impressus, in which 

 also the same smooth general appearance of the surface as in 

 V. quincuncialis is usually maintained, and for the same reason 

 as in that species. 



5. Ventriculites tessellatus. PL XIII. figs. 2, 3, 4. 



Membrane folded in regular quadrilateral and rectangular figures 

 usually more or less oblong : wall of moderate thickness. 



In Mr. Morris's Catalogue mention is made of a Ventriculites 

 quadratus, and reference is made to Goldfuss, pi. 33. fig. 1. I 

 have already* stated that this is no Ventriculite. The description 

 is, " seriebus pororum oblongorum rectis parallelis decussantibus," 

 — an error into which I cannot conceive it possible that Goldfuss 

 could have fallen if he really had the present species before him. 

 And the magnified sketch given in fig. 1 b of the same plate, — 

 unless fancy and not actual observation be there copied, — differs 

 as widely from any characters of the present species, and from 

 any form of Ventriculitic structure, as does the description which 

 has been cited. 



This form is rare ; and it is still rarer to find a perfect speci- 

 men. I have several fragments, excellently well displaying struc- 

 ture and fold, and during the summer of 1847 I was fortunate 

 enough to find a perfect specimen, with roots and margin, and 

 both body and cast, entirely perfect, and five inches in diameter ; 

 but I was so unfortunate as for it to break in pieces before I could 

 convey it home, a danger to which the collector of these delicate 

 fossils is peculiarly liable. Though this loss is not easily to be 

 replaced, the opportunity of verifying the entire form of the spe- 

 cies was important. The rarity of the species has prevented me 

 from making sufficiently extended observations on the moveable 

 processes to state, with confidence, anything definite in regard to 

 them. 



In this species the plaits may often readily be traced, with the 

 oblong depi'essions running along them on each face of the wall. 

 This being borne in mind, the figures often displayed on section 

 will readily be understood, when, instead of the regular alter- 

 nation of depressions seen at one place of section, as in fig. 4 



* Ante, vol. xx. p. 78. 



14* 



