THE CEPHALOPODA i2<) 



outwards from the umbilicus for half the height of the 

 lateral area, then to swing forwards, and to cross the 

 peripheral area with a very slight backward curve : they 

 thus contrast sharply with the radial lines, of which they 

 cross four or more in their course. 



3. Asteroceras obtusum (Fig. 39) is the index-fossil 

 of one of the zones of the Lower Lias. Specimens of this 

 or closely allied species are familiar to visitors to Lyme 

 Regis. The shell is of a light yellow-brown colour, but 

 is usually only preserved in fragments, the fossils being 

 essentially internal casts. The shell is spirally coiled in 

 a plane or bilaterally symmetrical spiral. Each com- 

 plete turn of the spiral is termed a whorl. The number 

 of whorls cannot usually be counted, the central being 

 hidden by rock-matrix which is difficult to remove, but 

 usually four or more can be counted : they become larger 

 rather rapidly from the centre outwards. The body- 

 chamber occupies rather less than half the last whorl : it 

 is filled with a grey matrix of argillaceous limestone, 

 while the gas-chambers are filled with brown crystalline 

 calcite. The visible part of the shell encircled by the 

 last whorl is called the umbilicus. Unlike that of gastro- 

 pods this forms a concave area on both sides equally. 

 The spiral line of contact between each whorl and the 

 next is the whorl-suture (corresponding to the suture of 

 a gastropod, but present on both sides of the am- 

 monite). 



A plane-spiral shell is measured as follows : the 

 diameter is measured from the outer edge of the aperture 

 (or if the shell is imperfect, of the latest part preserved) 



9 



