78 BRITISH PALEOZOIC ASTEROZOA. 



Measnrcments. — R : v : : Crb mm. : 3'2 mm. Breadth of arm at base is 2'6 mm. 

 Horizon and LocaUtij. — Lower IjucHow (Silurian) mudstones of Leintwardine, 

 Herefordshire. 



Family Piiomopal^asteuid.e, Schnchert. 



1914, Schuchert, C, Fossilium Catalogus, Animalia, pt, 3, p. 6. 



1915. „ „ Bull. 88, U.S. Nat. Mus., p. 73. 



" Progressive Phanerozonia with distinct columns of inframarginal plates. 

 Interbracliial areas more or less complex, composed either of inframarginals, 

 axillary interbrachials and ambulacrals, or of these with the addition of inter- 

 bracliial marginals and accessory interbrachials, or entirely of adambulacral plates. 

 Ambulacrals as a rule opposite, but they may also be slightly alternating. Podial 

 openings through the sutures in the lateral corners of the ambulacra! plates, but 

 proximally a few alternate pores may gradually pass medially, when there are 

 four columns of podial openings in each ambidacral furrow. Madreporite 

 abactinal. Abactinal plates very numerous, generally small, either in distinct 

 columns or rows or without either arrangement. The radial and supramarginal 

 columns may be very distinct or may be obscured as such. Accessory ossicles 

 always more or less abundantly developed." 



Schuchert divides the family into the three subfamilies of Mesopalaeasterinte, 

 Promopalfeasterinee, Anorthasterinae. 



Of these the Mesopalteasterince and the Promopalgeasteringe are related to the 

 Hudsonasterid^. One can clearly see the gradual development through the stages 

 named in the table on p. 65, from the small simple " Hiidsonaster " to the large 

 complicated " Fromopalsi'aster.'''' This change is narrated in detail by Schuchert 

 (85, p. 75) as follows : " Mesojialseaster apparently developed directly out of 

 IhKhondater in that the single axillary marginal plates [odontophors] are in the 

 former genns in nearly all the species pushed inward (interbrachial) and their 

 former place occupied by the proximal plates of adjoining inframarginal columns." 

 " Further, in HuiJsoiuister there are no abactinal accessory ray-plates, 

 but in Me.iOjMldeaster one or two more or less completely developed columns are 

 inserted on each side of the radial columns, or in other words, between the radials 

 and supramarginals. Between the inframarginal and supramarginal plates in Meso- 

 valseanlev there are also either a few incipient accessory plates, which in young 

 specimens are wholly absent, or there is a complete column of these ambital plates. 

 The accessory and axillary interbrachial plates are additions to the generic 

 structure of Hnchonaster and Pa/^as^er, and further distinguish Mesopalxaster from 

 both. This progression toward a greater number of columns of abactinal plates 

 attains its maximum in Promopalaiaster, and is most marked in P. luaf/nlficiis, 



