92 PICTURESQUE SKETCHES 



new species, the forms having become on the whole 

 more complicated. Higher groups, too, of radiated 

 animals were introduced, and the sea-urchin and the 

 star-fish, although rare, were not unknown in the 

 sea. Trilobites still remained, and herded together 

 chiefly in shoal water near the muddy and sandy bot- 

 toms ; and some other small crustaceous animals are 

 known from imperfect fragments of them occasionally 

 found fossil. 



But the shells of molluscous animals are too abun- 

 dant, too varied, and too widely distributed in the 

 rocks of the carboniferous period, to be passed over 

 without some careful notice. Those of Brachiopoda 

 (e. g. Spirifer, Productus, and Terebratula*) chiefly 

 preponderate, although not so much so as in the 

 older rocks, while the cephalopods, also very abun- 

 dant, are developed in new forms, many of them being 

 intermediate in their structure between the nautilus 

 and the ammonite, and others retaining the simple 

 form of the orthoceratite. 



The Productus (fig. 34) is a very remarkable and 

 interesting shell, although the exact nature of the 

 animal inhabitant has not yet been satisfactorily made 

 out. The shell is of a very fibrous texture, and a 



* Spirifer, Latin, spire-bearing ; so called because a coil of carbonate of 

 lime, useful probably in keeping the shell partly open, is not unfrequently 

 seen in the shells of this genus. An analogous contrivance is met with 

 in most members of the order of Brachiopoda in some shape or other. 



Productus, prolonged, or drawn out in length; one valve of the shell 

 being in most species prolonged beyond the other, and often to a great 

 extent. 



Terelratula, from terebratus, pierced; one valve being pierced at the apex, 

 to admit of the passage of a fibrous bundle proceeding from the other valve, 

 serving to attach the animal to some hard substance, as a stone or rock. 



