266 DENNIS, ON FOSSIL LIAS. 



culi few in number, rather tliick at their bases, tapering off, 

 branched, straggling, reticulate. 



I have examined other fossil fish, as the Dapedius and 

 others, but could find no appearance whatever of lacunae or 

 canaliculi in their structure. One, a very powerful-boned 

 fish, that I found myself at Lyme Jlegis, and which I believe 

 is at present undescribed, is entirely destitute. 



In the skull of the Pagrus, a recent sparoid fish, whose 

 jaws are armed with a most formidable array of canines and 

 molars, I have not been able to discover a vestige, either of a 

 lacuna or a canaliculus ; and this I should presume applies to 

 all the class. The sturgeon only shows them in its fin-bones 

 and dermo-skeleton, where the lacunae are very thick together. 

 Other large fish exhibited none. Our common roach has 

 them in the ribs. When, however, they are present, as in the 

 conger- eel, sturgeon, &c., they present precisely the same 

 characters, especially the very small number of canaliculi, and 

 their straggling, spider-like character. There is nothing 

 whatever to show that the bone under our consideration, as 

 the slightest examination of the figs. 21, 22, tells us, ever 

 belonged to a fish, for the lacuna? and canaliculi have no 

 icthic characters. The contest, therefore, runs between 

 saurians and mammals ; but I think I can satisfactorily show 

 that there are certain marked characteristics between the 

 lacunae and canaliculi of mammals and reptiles, and that in 

 these the fossil agrees with mammifers, though certain higher 

 saurians do approach the lower mammifers in apparent simi- 

 larity, as might be expected ; and much more irregularity of 

 structure is observable in the bone of the walrus or dugong 

 than we observe in the cat or giraffe. 



There are certain well-marked and distinctive differences in 

 the lacunae and canaliculi of Reptiles and Mammals ; and 

 which, when made apparent, render the task easy in de- 

 termining the question about a fossil bone, if its structure is 

 sufficiently preserved so as to show some of the lacuna? with 

 their canaliculi pretty perfect and well-defined. 



The lacunae in Reptiles are more generally irregular in 

 their shapes ; and by this I do not mean that irregularity 

 which is observable in some mammifers, where the lacunae 

 are some long, others oval, while some are nearly round ; for 

 a regularity is observed in one sense, as far as the individual 

 shape of a particular lacuna is concerned ; but in the reptile 

 the lacuna is more often irregular in its shape. We must 

 be careful, however, not to confound a transverse or tan- 

 gential section of an haversian canal with a lacuna. The 

 crocodile, which of all reptiles that 1 have observed, ap- 



