80 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



T. cocosanus, the types of which I have examined, differs from alaskanus in most 

 of its details. The abactinal and superomarginal spinelets are short, tubercular, 

 cylindrical and round-tipped; the crossed pedicellariae are much larger (0.38 to 0.45 

 mm. long); there are narrowly spatulate, small, toothed straight pedicellariae on the 

 sides of the ray (aud narrower-jawed ones on the furrow margin); the larger infero- 

 marginal spine is somewhat flattened and lanceolate, while the smaller is subtuber- 

 cular; the adambulacral plates are monacanthid, except about the first 10 and some 

 near the tip of ray which are diplacanthid; the oral plates are diplacanthid. 



T. fascicularis (Perrier) is a large species comparable in size with alaskanus (R 

 57 mm.; r 9 mm.). It differs in most of its details. The adambulacrals are dipla- 

 canthid and monacanthid, while the abactinal spinelets are shorter, stubbier, sub- 

 clavate, erect (0.55 to 0.65 mm. long), and are not arranged in evident transverse 

 lines except on the carinal and superomarginal plates. The abactinal crossed ped- 

 icellariae are about 0.36 to 0.38 mm. long. 



Genus AMPHERASTER Fisher 



Ampheraster Fisher, Ann. and Mag. Nat. Hist., ser. 9, vol. 12, 1923, p. 253. Type, 

 Sporasterias mariana Ludwig. 



Diagnosis. — Rays five, slender; disk small; abactinally resembling Pedicellaster ; 

 closely related to Tarsaster but differing therefrom in having large, narrowly or broadly 

 spatulate, prominently unguiculate, straight pedicellariae; an enlarged pair of inter- 

 brachial superomarginal plates overlapping a pair of firmly united sometimes enlarged, 

 corresponding inferomarginals, the quartet forming a very firm interbrachial skeleton; 

 no accessory inferomarginal spinelet; monacanthid adambulacrals (sometimes 

 partly diplacanthid); more open skeleton with larger dorsal meshes; a tendency to 

 warped, four-lobed superomarginals. Skeleton similar in essentials to that of Tar- 

 saster but in some species the superomarginal plates are in part three-lobed and in 

 part warped four-lobed, and there is a tendency to irregularity in the marginal 

 skeleton; postoral adambulacrals well separated or in partial contact, both conditions 

 being found sometimes in different ages of the same species; no actinal plates (except 

 an interradial rudiment; gonads opening dorsally, just above superomarginal plates; 

 tube-feet biserial throughout, quadriserial proximally, or quadriserial throughout. 



Remarks. — The difficulties encountered in classifyiug the species of this genus 

 have been indicated in the discussion of Tarsaster. 



The differences between marianus with its strong quadriserial tube-feet and 

 hyperoncus with biserial feet and pores may seem at first glance of generic value. 

 But A. chiroplus has characteristics of both species. Its proximal tube-feet are 

 quadriserial and its oral plates and first pair of adoral adambulacral plates are nearer 

 those of marianus. Its abactinal skeleton and three-lobed marginals resemble those 

 of hyperoncus. If these last characters are given precedence and chiroplus is ranged 

 with hyperoncus in a special subgenus, difficulty arises because the mouth plates and 

 enlarged first superomarginals of chiroplus are more like those of marianus (as are the 

 tube-feet), while a further complication arises from atactus which is not at all like 

 hyperoncus except in having three-lobed, and warped four-lobed, superomarginals. 



The character of the marginal plates is therefore of value only when taken in 

 combination with other features. It has been prominently used in the diagnosis 



