306 PALEONTOLOGY 



has twenty-eight major septa) the proto-septa are better 

 defined : in these species, however, the much greater 

 abundance of dissepiments tends to confuse the appear- 

 ance of a cross-section. 



Some species of Lithostvotion have cylindrical corallites 

 in a loose bundle, like L. irregularis ; in others (L. 

 basaltiforme) they are tightly packed together and become 

 polygonal (mostly hexagonal) in section. 



The essential feature of the genus is the laterally com- 

 pressed colurnella. Its species are confined to, and very 

 abundant in, the upper stage (Visean) of the Carbon- 

 iferous Limestone. 



3. Parasmilia centralis is a simple coral found in 

 the White Chalk of England. It is attached to molluscan 

 or echinoid shells by a spreading base, from which rises 

 a short cylindrical peduncular portion, soon expanding 

 into a conical form which may attain a length of 25 mm. 

 and a diameter of 12 mm. If further growth takes place, 

 the form becomes cylindrical ; a total length of 75 mm. or 

 more may be attained. The calyx is nearly circular, and 

 shows a prominent columella (which in a section is seen 

 to be spongy in texture), and forty-eight septa showing 

 almost perfect radial symmetry. Instead of being in two 

 series, major and minor, there are four series (cycles) of 

 unequal length. The first cycle consists of six long septa, 

 reaching the columella ; the second, of six, slightly 

 shorter, alternating with the first six ; the third, of twelve, 

 decidedly shorter, alternating with those of both first and 

 second cycles ; and the fourth of twenty-four, still shorter, 

 and alternating with all the others. This arrangement 



