DENNIS, ON FOSSIL LIAS. 267 



proaches nearer in the general appearance of its lacunae to 

 mammals, having its lacunae much more regular in form, is 

 still not free from the reptilian character. Great caution, 

 however, is required, as the walrus, and other lower mammals, 

 are inclined more or less to an iiTegularitj in this respect. 



The canaliculi of lizards are in general thicker than those 

 of mammals, and do not branch, but run in wavy courses from 

 the lacunae, where a number interlace. They hr.ve something 

 of the appearance of trellis-work. In the boa-constrictor the 

 saurian characters of the canaliculi are well displayed, which 

 more or less are apparent in all saurian bone tliat 1 have ex- 

 amined. The canaliculi also are much less numerous, though 

 their number varies in different Saurians. The definition, 

 therefore, of the general character of the lacunae and canaliculi 

 of Saurians will be : — lacunae more generally irregular-shaped 

 generally larger than mammals ; the canaliculi generally larger, 

 in a similar manner extending from all parts, not so branched, 

 long, thick and wavy. There are apparent exceptions, of 

 course, to this general definition ; but in the main it is truth- 

 ful and correct, llie irregularity of the lacunae in Reptiles 

 sometimes gives the appearance of branching ; but then a 

 thinner part of the lacuna is mistaken for a canaliculus. I 

 have observed this in the boa-constrictor. 



In examining mammal bone, the lacunae are observed gene- 

 rally to be (whether they are round, oval, or long and fusi- 

 form) of a pretty regular and uniform appearance, especially 

 in the higher mammifers, and are generally of an oval cha- 

 racter. They are also smaller than the generality of Saurians. 

 The canaliculi are viuch more mimerous than those of 

 Saurians, and often fork out at their base ; and besides this, 

 give out brandies. One end of a lacuna, when viewed with 

 its long axis, generally looks something like the head of a 

 stag's-horn beetle, the canaliculi being bifid, and branched 

 like antlers. In most of the lacunae of the crocodile, I have 

 observed the greatest apparent approach to this character ; 

 but it is more an apparent one {vide lacuna D). In the 

 mammal there is a considerable open space often left where 

 the extreme canaliculi bifurcate ; whereas in the crocodile 

 they simply radiate all round, with little or no bifurcation, or 

 run out into a long point. The mammal definition will then 

 be : — lacuna^ generally of regular form, most frequently oval, 

 bifurcate, with forked, branching, fine and numerous canaliculi. 

 These remarks and definitions refer to vertical sections, and 

 the lacunae are defined as they appear when seen with their 

 long axis in the field. Transverse or tangential sections give 

 the lacunae alwaj^s a more or less irregular form, and the la- 



