18 



have not yet been described. They are distinguished by their conical form 

 rising into an incurved apex, and by the first indication of a faculty pos- 

 sessed by some of this family of secreting a shelly plate or filmy approxi- 

 mation to one on their place of attachment. Of this we shall find a full 

 development in Hippony.r, and a still more perfect one in Calyplraa. The 

 British Pileopsis is mostly found attached to Oysters and Scallops. 



Species. 



1. astericola, Ad. fy R. 4. paleaceus, Meuke. 7. sagittiferus, Gould. 



2. crystallmus, Gould. 5. pilosus, Desk. 8. Ungaricus, Linn. 



3. militaris, Linn. 6. radiatus, Sars. 



Figure. 



Pileopsis Ungaricus. PI. 23. Pig. 138. Shell, showing its widely, 

 conical, cap-like form rising into an oblique incurved apex. 



Genus 2. HIPPONYX. 



Animal ; similar to that of Pileopsis. 



Shell ; obliquely cap-shaped, mostly supported on a shelly plate, 

 shell and plate both exhibiting horse-shoe muscular impressions. 



To any one accustomed to examine dead and broken shells for the sake 

 of the parasites to be found on them, Hipponyx will be a familiar genus. 

 The species both from the Eastern and Western seas of the tropical and 

 subtropical zones are numerous, but few of them have been described, and 

 the genus is much in want of the services of a monographer. There are 

 none in the European seas, and so little is the apparent zoological differ- 

 ence between this and the preceding group, so far as their characters are 

 known, that Hipponyx might fairly be merged into Pileopsis. The shells 

 are smaller, and mainly distinguished by the presence of a strong horse- 

 shoe muscular impression, which is repeated on the surface of the basal 

 plate* 



* " Of this remarkable genus Mr. Cuming brought home three species, in such perfect condition 

 ns respects the shell, as to possess both valves in situ. The two specimens which exhibit these 

 three species appear to me so interesting that I shall venture upon a particular description of 

 them. The first, of the species which I have named Hijip. Mitrula, is a group of about twenty 

 individuals, of various sizes, from ■£$ to i an inch in diameter, adhering by their lower or fiat 

 valves to an irregular piece of stone ; the attached valves, as usual, are conformed to the irregu- 

 larities of the surface of the stone, and when they have been at first attached to a cavity, they 



