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the gyroceran bend in this shell is sufficient to account for the dor- 

 sal furrow in the nephritic outline of the paranepionic substage. 



In the preceding remarks I have dealt solely with those genetic 

 series and forms in which the dorsal furrow appears, but there are 

 many in which there is no sign of any furrow. 



As has been said above, all of the straight and arcuate forms and 

 the gyroceran shells, in none of which has either a dorsal or a con- 

 tact furrow been found, except in Cranoceras. Passing these by, 

 one comes to the nautilian shells which are transitional between the 

 gyroceran and nautilian having the whorls in closer connection than 

 in the gyroceran and a slight contact furrow. These, so far as 

 known, have other correlative characters. The umbilical perfora- 

 tions are large and open, the apex of the conch is often free, the 

 contact taking place on the venter of the ana- or metanepionic sub- 

 stage or later, and the whorls are subsequently never involute, or in 

 other words, they are discoidal shells. 



In these shells I have in Paleozoic time found no exception to the 

 rule, that the dorsum of the nepionic stage is convex, and there is 

 no dorsal furrow, a furrow being formed only after contact, and in 

 later stages of development. One of the best examples of these 

 series is that of Barrandeoceras. Barrandeoceras Sternbergi contains, 

 as has been stated in the descriptions, two distinct forms, one Bar- 

 randeoceras Sternbergi, PI. xiv, Fig. 3, has the whorls approximate, 

 and in the other they are not in contact. The purely gyroceran 

 character of these shells is apparent in the loosely uncoiled 

 young as well as in the later stages. They are also valuable 

 in showing that the flattening of the dorsum and a dorsal lobe may 

 arise as in PI. xiv, Fig. 5, independently of contact, and this and 

 the form of the ephebic stage is precisely similar to that of the para- 

 nepionic volution of Barrandeoceras tyr annum. In this last and in 

 Bai'randeoceras Sacheri and Bohemicum, there is no dorsal furrow, 

 but these ephebic characters of Sternbergi are repeated and a contact 

 furrow is produced after the whorls touch. 



Estonioceras is another series in which nearly all degrees of coil- 

 ing can be studied, and here also the absence of a dorsal furrow in 

 the nepionic stage is a marked characteristic. The contact furrow is 

 maintained as long as the whorls are held together, showing pro- 

 gressive growth, but this rapidly disappears in the gerontic stage, as 

 shown by the figures given on PL v and vii. 



Remeleceras impress um is also a good example of the generation of 



