Geol.— Vol. I.] SMITH— LYTOCERAS AND PHYLLOCERAS. 1 39 



embryonic stage, but some division must be made, and it is 

 made here for convenience, although certainly a part of 

 the spiral tube was taken up by the embryo. There is a 

 remarkable constancy in the size and shape of the proto- 

 conch in all the later ammonoids; in all of them it is a 

 smooth, oval body, about one-half millimetre in diameter 

 and three-quarters in breadth. The protoconchs of the 

 earlier ammonoids were almost spherical, but the oval form 

 was soon developed and became a fixed stage, while other 

 stages that came later were entirely lost in ontogeny. This 

 is the only case known to the writer where ammonites in 

 their development skip over certain stages, but it is true of 

 nearly all later Mesozoic genera. In fact, two species of 

 the same genus often do not agree in omission of stages of 

 growth. Some species of Schlcenbachia, for instance, go 

 through in regular order all stages from Aiiarcestes up to 

 Glyphwceras ; others, on the contrary, omit all stages (so 

 far as any record of them in the shell is concerned) 

 between the protoconch and Glyphioceras, reaching that at 

 the second septum. The writer has noted that while dif- 

 ferent individuals of the same species are remarkably con- 

 stant in reaching the same stage at the same dimensions, 

 especially in the larval period, different species even though 

 very nearly related, usually differ very much in dimensions 

 at corresponding stages. This difference grows more pro- 

 nounced in the adolescent period, until decided specific 

 characters appear. In other words, specific difference is 

 merely exaggerated individual variation, and this in turn is 

 merely unequal acceleration or retardation of the appear- 

 ance of certain characters. All variation would seem, then, 

 to depend on Cope's law. 



The protoconch of Lytoceras alamedense (PI. XVI, figs. 

 I and 2) has a diameter of 0.56 mm., and width of 0.81 

 mm; in the protoconch is seen the pear-shaped siphonal 

 csecum or knob at the beginning of the siphuncle (PL XVI, 

 fig. 4). This is surely an embryonic feature, being pres- 

 ent before the development of the first septum, and prob- 

 ably represents the shrunken, horny covering of the nauti- 



