8o THE BLASTOIDEA 



FAMILY 1. ASTEROBLASTIDAE. Protoblastoidea with an indefinite 

 number of thecal plates, bearing diplopores, and with a pore-plate adjacent 

 to, but distinct from, the deltoid in each interanibulacrum. Genus 

 Asteroblastus, Eichwald (1862), emend. Schmidt (1874), Ordovician, Russia 

 (Fig. I.). Theca pentagonal, on a relatively small round stem ; its hemi- 

 spherical dorsal portion is composed of 4 BB and 5 RR, between which are 

 from twenty-five (A. Volborthi) to fifty (A stellatus, type-sp.) polygonal 

 plates; all plates have radiating ribs ("axial folds"), between which are 

 diplopores. Round the mouth are 5 A, between which food-grooves 

 pass to the ambulacra. Each ambulacrum consists of two rows of side- 

 plates (adambulacrals), set alternately on either side of the food-groove, 

 branches from which run to right and left between adjacent side-plates. 

 Any underlying plate there may be is entirely covered by the side-plates. 

 At the ends of the grooves are sockets, by which uniserial brachioles were 

 attached .to the side-plates. Grooves down the brachioles joined the food- 

 grooves of the ray, and all probably were covered with small alternating 

 plates (ambulacrals). Adjoining each A and 

 separating the adjacent ambulacra, is a large 

 diplopore- bearing plate ; in A. tuberculatus 

 this is much like other plates of the theca 

 (on which ground Haeckel has separated the 

 species as Asterocystis, Fig. II.) ; but in A. 

 stellatus and A. Volborthi it is enlarged, has 

 a pronounced median crest, and is almost 

 separated by the ambulacra from the ordinary 

 thecal plates. [The anus appears to have 



Portion of the oral surface of been at the distal end of this " pore plate " 

 :T^eS^sun1: in one f ^ interambulacra, which is wider 



ing the peristome ; s.p, side-plates than the Others.] FAMILY 2. BLASTOIDO- 



CEINIDAE. Protoblastoidea with a definite 

 number of thecal plates, without diplopores, 

 and without distinct pore- plate. Genus Blastoidocrinus, E. Billings 

 (1859), Canadian, and Schmidt (1874), Russian, Ordovician, is only 

 known from imperfect specimens, so that only the following details can 

 be given (Fig. III.) : The small round stem is inserted into a consider 

 ably invaginated base, as is the case in several crinoids. BB, at least 3. 

 RR, 5, not notched for reception of ambulacra, but with truncate upper 

 margin ; they support 5 large interradially situate A. Each A consists of 

 two parts : an orad, subtriangular piece, like that called deltoid in Astero- 

 blastus, and a lower, triangular piece corresponding to the pore-plate in 

 Asteroblastus ; the connection between the two parts is still slight ; indeed, 

 in Canada the orad portions are often fused with one another into a pen- 

 tagonal frame. The lower portions show a marked striation, due to stroma 

 strands, at right angles to the radio-deltoid suture, and on the sutural 

 edge the stereom is thrown into folds (incipient " hydrospires ") which, 

 however, have not been observed to pass into the RR. The food-grooves 

 pass out between the A ; the lancet-plat* supporting them is covered by 

 alternating side-plates, which receive branches from the main groove, and 

 are provided with a clear brachiole-facet ; the remains of brachioles are 



