250 THE STELLEROIDEA 



ambulacral ossicles became narrower, greater freedom was gained by the 

 opposition of the two plates of a pair. The artificial nature of the 

 divisions based on this character is shown by Stiirtz's action in dividing 

 several of his families into halves and placing members of the same 

 family in two different orders. Thus he founded a family Palajechin- 

 asteridae for the genera Eckinasterella and Palasteriscus ; but he included 

 the latter among the division with alternate ambulacral ossicles, and the 

 former in the division in which these ossicles are opposite. As the 

 " Palocasteroidea " include representatives of both Phanerozonate and 

 Cryptozonate Asteroids, and of several families of each, it is necessary to 

 dismember such an artificial group. 



FAMILY 1. PALJEASTERIDAE. Phanerozonia with most or all of the 

 ambulacral ossicles alternate ; the madreporite is dorsal. Oral armament 

 adambulacral. Abactinal skeleton tessellate. Rays long, disc small. 

 This family includes a series of Asteroids occurring in the Lower 

 Palaeozoic. In most of them the ambulacral ossicles are alternate, 

 but in some cases these plates are opposite, either for a part, or for the 

 whole length of the arm. Hence this character does not seem to be of 

 the value assigned to it. The marginal ossicles are always conspicuous, 

 and, as far as is known, the madreporite is fairly small and dorsal in 

 position. The oral skeleton consists of a ring in which the adambulacral 

 plates are most conspicuous. There are two sub-families. SUB-FAMILY 1. 

 PAL^ASTERINAE, in which the ambulacral ossicles are alternate. Genera 

 Palwaster, Hall; Argaster, Hall; (?) Monaster, Eth. jnr. ; and (?) 

 Pctraster, Billings pars. The genera are all Palaeozoic, ranging from the 

 Cambrian to the Devonian. Among existing families the nearest ally is 

 the Archasteridae. SUB- FAMILY 2. XEN^STERINAE, including those 

 with inooi, of the ambulacral ossicles in opposite pairs. Genera Xcnaster, 

 Simonowitsch ; Tetraster, Eth. jnr. & Nich. 



FAMILY 2. PALJEASTERINIDAE. Phanerozonia with alternate ambu- 

 lacral ossicles and small marginal plates. The oral armature is adam- 

 bulacral. The madreporite is abactinal. The rays are short and are 

 separated by large interradial areas. Genera Palaasterina, M'Coy ; 

 tichoenaster, Meek & Worthen. 



FAMILY 3. ASPIDOSOMATIDAE. Phanerozonia with alternate ambula- 

 cral ossicles ; large marginal ossicles and large interradial areas ; rays 

 massive, petaloid, sub-petaloid, or tapering. On the abactinal surface 

 there are large depressed areas between the marginal ossicles and the 

 outermost of the longitudinal series of large plates which run along the 

 arms. Genera Aspidosoma, Goldf. (the type-species is A. Arnoldi, but 

 as this is imperfectly known, A. pdaloides, Simonowitsch, may be 

 accepted provisionally) ; Palaeonedria, Stiirtz ; Palaeostella, Stiirtz ; Trich- 

 asteropsi^ Eck ; Archanterias, Mull., may belong here, but the genus is 

 insufficiently known. 



FAMILY 4. TAENIASTERIDAE. Phanerozonia with alternate ambulacral 

 ossicles. There are neither disc nor interbrachial areas. The adambu- 

 lacral plates are large and act as marginal plates. The axes of the 

 marginal plates are parallel and the rays petaloid (as in Stenaster), or the 

 axes of the marginal plates are convergent ; these plates bear spines on 



