86 
PROaRESS OF MICROSCOPICAL SCIENCE. 
however, the young tooth sacs have a connection through a band of 
epithelial cells. The first process is a dipping down of a narrow 
process of the oral epithelium, the extremity of which, after it has 
penetrated in some, as the snake, to a great depth, becomes dilated, 
and is transformed into the enamel organ ; and this is the case whether 
a recognizable coat of enamel is or is not to be found on the perfect 
tooth. Subsequently to the dipping in of the band of epithelium, and 
concomitantly with the dilatation of its end, a dentine pulp is formed 
opposite to it. This may constitute the entire tooth sac, which is 
then wholly cellular, as in the newt ; or it may go on further to the 
formation of a connective-tissue tooth capsule. The external thin 
structureless coating of the teeth of Ophidia is derived from an unmis- 
takable enamel organ, developed as above described ; it is therefore 
enamel, and not cementum, as it is denominated by Professor Owen. 
The successional tooth sacs, very numerous in the snakes, are located 
in a sort of capsule : this character, peculiar to the Ophidia, and most 
marked in the lower jaw, is of obvious service during the extreme 
dilatation which the mouth undergoes, as is also the tortuosity of the 
process of epithelium, before it reaches the collection of tooth sacs. 
The epithelial band may be traced winding by the side of the older 
tooth sacs till it reaches the position of the youngest, where it ends 
in a caBcal extremity, to be transformed into the enamel organ next 
developed. In fine, the stages of open groove, free papillae, and 
encapsulation of the same, have no existence whatever in Batrachia 
and Eeptilia, their existence having been previously disproved in 
Mammalia. 
New Species of Hhizopods. — In ' Silliman's American Journal ' for 
January, 1875, there is an account of a recent paper by Professor 
Leidy on the above subject, which was read before the Philadelphia 
Academy of Natural Science. Professor Leidy says that among the 
amoeboid forms noticed by him in the vicinity of Philadelphia, there 
was one especially remarkable for the comparatively enormous quan- 
tity of quartozse sand which it swallowed with its food. The animal 
might be viewed as a bag of sand ! It is a sluggish creature, and 
when at rest appears as an opaque white spherical ball, ranging from 
^ to f of a line in diameter. The animal moves slowly, first assuming 
an oval and then a clavate form. In the oval form one measured f of 
a line long by f of a line broad, and when it became clavate it was 
§ of a line long by of a line broad at the advanced thick end. 
Another, in the clavate form, measured |- of a line long by J of a line 
wide at the thick end. The creature rolls or extends in advance, while it 
contracts behind. Unless under pressure it puts forth no pseudopods, 
and the granular entosarc usually follows closely on the limits of the 
extending ectosarc. Generally the animal drags after it a quantity of 
adherent dirt attached to a papillated or villous discoid projection of 
the body. 
The contents of the animal, besides the granular matter and many 
globules of the entosarc, consist of diatoms, desmids, and confervaB, 
together with a larger proportion of angular particles of transparent 
and mostly colourless quartz. Treated with strong mineral acids so 
