1314 THE VOYAGE OF H.M.S. CHALLENGER. 



Family LXV. P odo c yktid a, n. fam. (PL 67, 68, 71-73). 



Theopilida et Theoperida, Haeckel, 1881, Prodromus, p. 435, 436. 



Definition. — T r i c y r t i d a triradiata. (Cyrtoidea with a three-jointed shell, 

 divided by two transverse constrictions into cephalis, thorax, and abdomen, and bearing 

 three radial apophyses.) 



The family P o d o c y r t i d a, composed of the Theopilida and Theoperida of my 

 Prodromus, comprises those Cyrtoidea in which the lattice-shell is three-jointed and 

 triradial, bearing three external apophyses. The two subfamilies differ in the shape of 

 the basal mouth, which in the Theopilida is a simple wide opening, in the Theoperida 

 closed by a lattice-plate. The latter are derived from the former by development of this 

 closing plate. The phylogenetic origin of the Podocyrtida may be found in the 

 Tripocyrtida, from which they are derived by development of an abdomen. But there 

 are also some forms, which may be derived directly from the Plectoidea, and the 

 origin of some other forms seems to be doubtfuL 



The Podocyrtida represent one of the largest groups of Cyrtoidea, very rich 

 in numerous species, which are partly very common and widely distributed. A large 

 number of living and fossil species has already been described and figured by Ehrenberg, 

 (1872, 1879, loc. cit.). These mainly belong to his genera Podocyrtis, Pterocanium, 

 Rhopalocanium, Lithornithium, &c., and many of them are very characteristic and 

 common forms, in which the triradial structure of the three-jointed shell is visible at 

 first view. We distinguish here twenty genera and one hundred and fifty species. 

 The majority have a shell with a simple wide open mouth (Theopilida, fourteen genera 

 and one hundred and eighteen species), in the minority the mouth is closed by lattice- 

 work (Theoperida, six genera and thirty -two species). 



The three joints of the shell have in the majority of Podocyrtida such a proportion 

 that the cephalis is the smallest, the abdomen the largest, and the thorax between them 

 intermediate in size. The cephalis bears almost constantly an apical horn ; this is 

 rarely reduced or lost ; sometimes two or more horns are developed. The three radial 

 apophyses arise originally from the base of the cephalis, and may from this point run 

 along the shell-wall embedded in the lattice-plate of the thorax and the abdomen. They 

 leave the latter at very different points, and form either lateral wings or terminal feet, 

 sometimes both together. They are either solid or latticed, sometimes also branched, 

 and of very various shape. Often only the three terminal feet surrounding the mouth 

 have remained, whilst the ribs have disappeared. 



Many species of this large family are very variable and connected with other 

 different species by numerous transitional forms ; the distinction of the genera described 

 is also often very difficult. 



