64 SMITHSONIAN INSTITUTION 



This plan of anal structure began in the Ordovician, and is found in the 

 Taxocrinoid genera from Protaxocrinus and Gnorimocrinus in the Trenton 

 and Silurian, to Taxocrinits and Onychocrinus in the Carboniferous, surviv- 

 ing until the Kaskaskia. Being thus so characteristic of that series of genera, 

 it may well be called the Taxocrinus plan. It has persisted to the present time, 

 as shown in the early stages of Antedon, Comactinia, Promachocrinus, etc., in 

 which a plate directly connected with the tube is progressively developed, while 

 in the adult of these forms, and of Calamocrinus, Ptilocrinus, Phrynocrinus 

 and Thallassocrimis , plates of anal origin have completely disappeared from 

 the dorsal cup. 



2. The second plan is that of a simple extension of the anal system by 

 the addition of other plates of a solid nature similar to that of the original 

 plate. These plates are joined by sutural union to the adjacent plates of the 

 posterior rays, and also with each other ; so that as far as they extend upward 

 they are incorporated into the calyx walls in the same manner as the plates of 

 the other interbrachial areas. The posterior basal is either simply truncate, 

 followed by one plate ; angular, followed by two plates ; or truncate with slop- 

 ing shoulders, followed by three plates ; but in each of these cases all the plates, 

 and others of a similar nature succeeding them, form by sutural union (of 

 course of the loose order characteristic of the whole group) part of the calyx 

 wall. 



This form of anal structure existed from the Silurian to the Carbonifer- 

 ous. The primitive stage is found in such genera as Lecanocrinus (PL I, 

 fig. 34), and Anisocrinus (PI. X, fig. 3b), in which the posterior rays arch 

 over the anal plate, leaving no vacant space above it. In Calpiocrinns we see 

 its simplest extension by the addition of single plates in succession (PI. VIII, 

 fig. 2b). In Temnocrinus we have a further modification by the addition of 

 two or three plates abutting on the anal (PI. XVI, figs. 4, 5). The develop- 

 ment was rapid, for in another Silurian genus, Sagenocriiius, in which the 

 dextrorse asymmetry caused by the presence of a radianal is still a feature, we 

 have a perfect example of this plan in its fullest extension, where the anal area, 

 following upon an angular posterior basal, is completely filled to the height of 

 several orders of brachials with solid plates forming a regular continuation 

 of the calyx wall. Because of its remarkable development in that genus this 

 form of anal structure may appropriately be called the Sagenocriiius plan. It 

 is carried forward with equal perfection into the Carboniferous by the genus 

 Forbesiocrinus, in which the radianal has risen to the upper oblique position; 

 and it persists through the Keokuk and Warsaw to the Coal Measures. 



While there are some genera from the Silurian to the Carboniferous in 

 which the calyx seems to be bilaterally symmetrical, yet it is a fact that the 

 dextrorse asymmetry remains as a general characteristic of this form as well 



