MOLLUSC A— CEPHALOPODA. 19 



whorls actually come into contact (see Hyatt, " Phylogeny of an 

 Acquired Characteristic"). 



As long as an impressed zone exists, the shell is spoken of as a 

 nautilicone {Nautilus in the broad sense) ; when the whorls are 

 barely in contact, without being impressed, or when they coil in 

 a plane without contact, it is spoken of as a gyroceracone (Gyro- 

 ceras of early authors) ; when the tube is merely bent, without 

 making a complete revolution, it is a cyrtoceracone (Cyrtoceras 

 of early authors) and when it is straight it is an orthoceracone 

 (Orthoceras of early authors). Many nautilicones pass through 

 a gyroceraconic and even cyrtoceraconic stage in their own devel- 

 opment, while gyroceracones are cyrtoceraconic in their early life 

 history and cyrtoceracones when young are orthoceracones. Among 

 the Ammonoidea the terms ammoniticone, mimoceracone and hac- 

 triticone are sometimes used for the close-coiled, loose-coiled and 

 straight (primitive) forms. In some cases, both among Nauti- 

 loids and Ammonoids, coiling may be in an asymmetric spire 

 resembling the gastropod shell. This is spoken of as a trochocera- 

 cone among the Nautiloids (Trochoceras of older authors), and 

 turriliticone among the Ammonoids (Turrilites of older authors). 

 In old-age individuals of both groups and in adults of decadent 

 series, the last portion of the whorl often becomes free, or even 

 straight again (Lituites of authors among Nautiloidea, Scaphites 

 and B acuities among Ammonoidea). Such decadent forms among 

 ammonoids assume a variety of form. The last whorl may curve 

 to a greater radius, generally with a subsequent abrupt retral 

 curve (scaphitean) , or wholly straight except for a minute initial 

 coil (baculitean) ; it may coil in a loose spiral throughout (crio- 

 ceran), or with a final straight portion and subsequent retral curve 

 (ancyloceran) ; it may consist of two or more straight "limbs" 

 connected by abrupt curves separated (hamttean) , or close in con- 

 tact (ptychoceran), or it may become an irregularly twisting tube, 

 generally with loss of ornamentation (heteroceran). 



The initial point or protoconch of cephalopod shells is bulb- 

 shaped, calcareous and generally preserved in the Ammonoids, but 

 non-calcareous and generally lost in the Nautiloids, where its 

 former presence is often indicated by a scar at the apex of the 

 shell proper. 



One of the characteristic features of the cephalopod shell is its 



