RELATIONS OF HELDERBERGIAN FAUNA 277 



Triplegia; of the orthoids, Platystrophia ; of the pentameroids, Capellinia, 

 Pentamerus, and Oonchidium; of the spire-bearing families, Meristinay 

 Whitfieldella, Hyattella, Olinlonelbt, Hindclla, and Hommospira — and Eich- 

 waldia or Dictyonella, Parastrophia, and Rhynchotreta fail to have a single 

 representative in the Helderbergian or any younger formation. 



The gastropods of the Helderbergian undoubtedly indicate the Devonic, 

 in the prolific development of the platyceroids of which there are about 

 38 species. They are very closely linked with the Oriskany forms. Even 

 the Siluric genera Strophostylus and Dlaphorostoma (= Platyostoma) show 

 progression in their larger species. 



The pelecypods are rather scarce, but the abundance of Aviculopec- 

 tenidse points rather to the Devonic because of their paucity in the Siluric. 

 The Devonic genera Actinopteria and Paracyclas begin here. Other genera 

 with un described species — Grammysia, Goniophora, Orthonota, Nuculites, 

 and Maminka — are present. The specimens are in the Beecher collec- 

 tion in Yale University Museum. 



In the abundance of fenestelloids and fistuliporoids, the Bryozoa are 

 greater than in the Siluric, and more in harmony with the younger 

 Devonic faunas. 



The crinoids of the Helderbergian are often fragmentary and for that 

 reason the following genera may be dropped from this review : Aspido- 

 crinus may be based on roots, and probably also Camarocrinus, while 

 Brachiocrlnus, or rather Herpetocrinus, is only known from fragments. The 

 other genera found here are Homocrinus and Melocrinus, ranging from the 

 Siluric to Middle Devonic ; Mariacrinus, Cordylocrinus, and Marsupio- 

 crinus, found either in the Ordovicic or Siluric, and Edriocrinus in the 

 Oriskany and Middle Devonic. Thus far the evidence points to a Siluric 

 facies for the crinoids. However, the negative evidence — that is, the 

 absence of characteristic Siluric genera — Ichthyocrinus, Lecanocrinus, 

 Macro stylo crinus, Glyptastcr, Dimerocrintis, Lavipterocrinus, Lyriocrinus 

 E iiccdyp to crimes ,Pisocri mis, and Stephanocrinus — furnishes indications that 

 the crinoid development of the Helderbergian is markedly varied, and 

 not Siluric in character. Of these genera but two — Lecanocrinus and 

 Eticalyptocrinus — recur in the Devonic of Germany, each with one species. 



From the foregoing summary of the Helderbergian fauna it is evident 

 that most of the characteristic Siluric genera of trilobites, brachiopods, 

 and crinoids are there absent. This might be expected, for, as has been 

 seen in the previous chapter, about 2 per cent of the Helderbergian 

 fauna are derived from the Siluric. On the other hand, in some of the 

 trilobites, Biyozoa, and pelecypods, many of the gastropods, but more 

 particularly in the diversified brachiopods, are met organic groups, 

 which in their culmination are characteristic of the Devonic. It can 

 not be denied that the Helderbergian fauna has a Siluric facies, yet 



