SILURIAN PERIOD 33 



kept clear of the breathing water (JPleurotomanidce, Bellero- 

 phontidcB). 



The only new forms of importance possessed long turreted 

 shells with well-defined spires (MacrocMlina, Loxonema). 

 The MelaniidcB of our own day may be descendants of these 

 sea-snails. If such be the case the family, in course of time, 

 must have migrated from the sea ; for its members now 

 live in fresh water only. 



Scaphopods or " tooth-shells " had undergone some slight scaphopods 

 structural changes. They were, however, wellnigh sufficiently 

 modified for perpetuation through the ages ; for present- 

 day scaphopods differ but little from them {DentaUum). 

 In spite of excessively sluggish habits they have managed — 

 unhke some molluscs — to retain their heads. Hearts never 

 seem to have been a scaphopod property. 



Nautiloids in more or less straight shells were still abundant, cephalopods 

 There were also new species of these old-fashioned forms ; the 

 shells having undergone modifications in the chamber- 

 partitions, or in the position of the tube which connected 

 up the various chambers. Huge animals with excessively 

 ponderous shells, such as had appeared in Ordovician times, 

 had apparently died out. Life no doubt had proved unequal 

 to the burdens which gluttony had entailed. Straight- 

 shelled nautiloids of considerable size were, however, still 

 conspicuous in marine life (Orthoceras annulatum, etc.). 



Some peculiar forms were now appearing boxed up in egg- 

 like shells with a T-shaped aperture. Through this contracted 

 opening the " arms," no doubt, could be thrust ; but the rest 

 of the animal must have been held close prisoner (Gom- 

 .phoceras). This seclusion doubtless had a protective value ; 

 but in the end the drawbacks proved greater than the ad- 

 vantages. Nautiloids, however, thus sequestered, were in 

 evidence during the two succeeding Periods. 



The forms with coiled shells were making a decided advance 

 in numbers. Those in shells of the transitional pastoral-staff 

 pattern were certainly on the wane : but there were many 

 new species in shells more or less of nautilus-shape (Herco- 

 ceras, Trochoceras, Ptenoceras). The coiled -shell forms 

 were destined in time to supplant those in shells of older 



D 



