No. 432.] STUDIES OF GASTROPODA. 921 
primitive condition of a coiled protoconch — is lost. From 
the characters of the initial whorls of the protoconch we may 
argue that the radicle of the coiled gastropods must have been 
a naticoid type with a well-marked umbilicus. Such a type 
is found in Straparollina remota Billings, one of the earliest 
coiled gastropods of the Etcheminian or Lower Cambrian of 
the Atlantic border province of North America. That it is 
probably not the most primitive type of gastropod is suggested 
by the consideration that the earliest stage (ana-phylembryonic) 
of the protoconch is not coiled, but rather cap-shaped like 
modern Patella. Such primitive types are found in Lower 
Cambrian species which have variously 
been referred to Platyceras, Scenella, 
or Stenotheca, owing to the want of 
sufficient characteristics to define their 
exact relations. 
Our modern patelliform shells are 
probably not primitive types, as shown 
by the protoconchs of Acmza and 
Crepidula, which are coiled. Tem : 
Ornamentation of the Protoconch.— Pe s Midi diu X4. med 
In a number of species of gastropods vs icee ep esc A aid 
the later whorls of the protoconch are the abruptly beginning, accelerated 
ornamented by riblets (Figs. 1, 5), and Sue | developes 
more rarely by true revolving lines or 
spirals. In others a carina appears on the whorls of the 
. protoconch.! These structures normally belong to the conch, 
Where their strength and variation constitute the chief features 
by which the whorls are characterized. Their occurrence on 
the protoconch is due to a backward pushing of the normal 
conch characters, until they appear on the final whorls of the 
protoconch. This is in conformity with the law of tachygenesis, 
or acceleration in development, which is the key to the under- : 
standing of the taxonomic values of shell characters in 
1 For an account of the characteristics of the protoconchs of a pep of 
gastropods, see Baker, F. C., On the Modification T wo r IX 
alio Prec PE Ami pm (1890), pp. 66-72; (1894), pp. 223-224; Roch. Acad. 
Sez. (1891), pp. 130-132. 


