254 ECHINODERMATA— ASTEEOZOA phtlum iv 



Position at the points of origin of the arms, are called radial shields 

 (Fig. 359). On the ventral surface of the disk, the inner angle of each 

 interbrachial space is occupied by a single large plate termed the mouth 

 shield {scutum buccale) (Fig. 360), one of which serves as the madreporic body. 

 But in the Cladophiuroida the mouth shields are often feebly developed, or may 

 be wanting altogether ; and in place of them a madreporite is f ound in one or 

 all of the interrays. The mouth shields are bounded proximally by a pair of 

 somewhat smaller plates called the side mouth shields. Finally, within the 

 side mouth shields, and usually pressing against them, are the jaws which are 

 sometimes covered by the skin or by granulations (Fig. 360). Teeth are 

 constantly present, being attached to the jaw-plates by small muscles, and 

 other tooth-like processes (tooth-papillae and oral-papillae) are generally present 

 at the inner angle or along the sides of the jaws. 



A natural Classification of the Ophiuroidea remains to be established. 

 Those who have worked principally on Eecent forms have not, as a rule, 

 proposed any completed System ; and so while our knowledge of the number 

 and variety of Eecent species has increased enormously, no progress has been 

 made toward a rational arrangement of the class. On the other band, some 

 valuable work by paleontologists has been vitiated by ignorance of the Eecent 

 forms, while the difficulties of the material with which such work must be 

 done has led to radical differences of Interpretation and opinion. 



The proposed groups Protophiuroidea and Euophiuroidea may be natural 

 divisions, but as the character upon which the class is differentiated is the struc- 

 ture of the arm and the development of " vertebral ossicles," the Classification 

 proposed by Bell and elaborated by Gregory may better be adopted as a 

 basis for further study. Under this system four Orders may be recognised, 

 but family limits are uncertain and unsatisfactory. The termination of 

 Gregory's ordinal names is altered to end in -oida. 



Order 1. LYSOPHIUROIDA. 



Ophiuroidea in which the vertebral ossicles are incomplete, the two halves not 

 being united, but separate and alternate. There are no ventral arm-plates ^nd thus 

 a more or less distinct ambulacral furrow is present. 



This Order includes a group of Paleozoic Asterozoans, intermediate 

 between Ophiurans and Starfishes. They differ from the latter only in the 

 general form, the arms being sharply set off from the disk, but probably the 

 alimentary canal and reproductive organs were confined to the disk. The 

 characters of the ambulacral plates are often uncertain, but they may be 

 either subquadrate or " boot-shaped." The character of the mouth-parts in 

 this Order and the next has been well worked out by the Sollases, and their 

 primitive character clearly shown. The principal genera are Protaster Forbes 

 from the Silurian, and Bundenhachia and Palaeophiura Stürtz, from the Devonian 

 of Europe. The Ophiurans from the Lower Silurian strata of Bohemia, 

 whose structure is discussed by Jaekel, are undoubtedly members of this 

 Order. The most important genus is Bohemura. 



Order 2. STREPTOPHIUROIDA. 



Ophiuroidea in which the vertebral ossicles are more or less complete, and in 

 any case, the two halves are opposite. The ossicles articulate with each other by ball- 



