THE CYSTIDEA 



75 



Thecal plates clearly differentiated into : (a) smooth, irregular, and 

 depressed interambulacrals ; (b) transversely elongate adambulacrals. 

 Diplopores at right angles to main food -groove, and confined to inner 



Fio. XLV. 



Protocrinus oviformis, after Volborth. 1, oral surface, showing food-grooves partly covered 

 by ambnlacrals, x jj ; 2, aboral surface of young individual, showing stein -attachment; 3, 

 aboral surface of old individual, without stem. 



portions of adambulacrals. Each adambulacral bears a brachiole-facet ; 

 there are about thirty-six in each ray. 



FAMILY 4. MESOCYSTIDAE. Diploporita in which the food- grooves 

 extend over the theca almost to the aboral pole, and are regularly 

 bordered by alternating brachioliferous adambulacrals, raised above and 

 outside the adjacent interambulacrals. Diplopores confined to 1 inter- 

 ambulacrals. Five interradial deltoids (A) sur- 

 round the peristome. In this family we reach 

 the final stage of the Diploporita, although a 

 branch parallel with the Mesocystidae passes on 

 in the direction of the Eublastoidea, beyond the 

 boundary of the Cystidea. Genera Mesocystis, 

 Bather (Jan. 1898, =Mesites, Hoffmann, 1866; 

 Nikitin, 1877 ; Agelacrinus, Schmidt, 1874), 

 Ordovician, Esthonia. Theca (Fig. XLVII. 1) 

 simulates that of a regular echinoid, or still more, 

 Edrioaster, since the mouth is on the upper 

 surface ; from it narrow food-grooves, protected 

 by covering -plates, pass straight down to the 

 margin of the flattened aboral surface (Fig. 

 XLVII. 2). Adambulacrals raised above the (Amb) bordered by adambu- 

 general surface of the theca, by the pushing in JS^er Sef" 18 

 under them of the adjacent interambulacrals ; 



thus they outwardly resemble the sub-ambulacrals or side -plates of 

 Callocystinae (Fig. XLVII. 3). The interambulacrals do not, however, 

 absolutely meet underneath the adambulacrals, but leave an irregular 



Fio. XLVI. 



