i^4 THE AMfimCiAlSf MOiST'THLY [Aug. 



(probably illusory), but as they mature this feature be- 

 comes more prominent by the enlargement into bead-like 

 cavities, which are sometimes contiguous, at others slight- 

 ly separated. The filaments are rather large in diame- 

 ter for the size of the plant, and they have a tendency to 

 erode the surface of the shell. Devoid of appendages. 

 Very rare. Typical specimens have been found in recent 

 material from Lagos Bay, S. Australia, and in Miocene 

 deposit from Auckland, New Zealand. Fig. 6 x 300. 



Achlya gracilis. — The distinguishing characters of this 

 species appear to be more uniformly persistent than in 

 many of the members of the group. The filaments may 

 be divided into three distinct group*-; which always oc- 

 cupy separate portions of the shell they invade. The 

 contour of the plant is displayed at or near the surface of 

 the nacreous layers ; at frequent intervals branches are 

 given off which descend vertically into the shell; in some 

 instances these terminate in irregular-shaped cavities (to 

 which further reference will be made), while others pass 

 on until they sometimes reach the opposite surface, where 

 they may terminate, or they may be deflected and return 

 for some distance towards the original point of entry. 

 These filaments may be simple and hair-like, or they may 

 be slightly branched. The superficial aspect of the plant 

 conveys the idea of rigidity, caused by the uniformity 

 with which the branches divide. The filaments are ex- 

 ceedingly small in diameter, and at the point where they 

 descend into the shell they appear to be jointed. The 

 vesicles which terminate the short branches are generally 

 contained wholly within the nacreous portion of the 

 shell ; in their perfect condition they are spherical, and 

 are provided with an excretory duct, but they appear to 

 speedily erode the shell and form irregular-shaped cavi- 

 ties ; with the exception of the excretory duct referred to, 

 the plant appears to be destitute of appendages for com- 

 municating with the surrounding water ; in this respect 



