Limitation of Genera among the Hydroida. 367 
cemiform peduncles, which spring from the body of the polypite 
between the proximal and distal verticil of tentacles. 
The genus Tudularia of modern authors has been broken up by 
Agassiz into four separate genera, for one of which he retains the 
older name Tudularia, while he designates the three others respect- 
ively Parypha, Thamnocnidia, and Ectopleura (Cont. Nat. Hist. 
U. S. vol. iv.). He gives no technical diagnosis 'of any of these 
genera; but, from my own knowledge of the European species which 
he separates from Tudularia, as well as from the very detailed © 
descriptions and beautiful figures of the American species which he 
now for the first time describes and refers to his new genera, I can 
find only in one of these forms (namely, the Tubularia Dumortiert 
of Van Beneden) characters which would, in my opinion, justify the 
proposed dismemberment. For Tudularia Dumortiert Agassiz con- 
stitutes a new genus under the name of Hetopleura, and in this I 
willingly follow him ; but the only character of importance by which 
Parypha and Thamnocnidia are separated from Tudularia is the 
non-development of gastro-vascular canals in the sporosacs of the. 
species referred by Agassiz to these genera, while they are found in 
the sporosac of Tubularia indivisa. 
Now I cannot admit that the presence or absence of these canals 
in a sporosac, so long as they do not bring with them the develop- 
ment of an open contractile umbrella capable, when detached, of 
acting as a swimming-organ, can be regarded as affording a character 
of generic value, even though we leave out of view the great difficulty 
of detecting it, which is a practical rather than a scientific objection. 
_ Again, between Parypha and Thamnoenidia the only difference 
alleged is in the structure of the tentacula-like tubercles which occur 
upon the summit of the sporosac. I believe, however, that there is 
here no important difference. I have carefully examined the sporo- 
sacs of Tubularia coronata, Abildg., a species which Agassiz refers 
to his genus Thamnocnidia, and I can find no essential difference 
between the tentacular tubercles which crown the sporosac in this 
species and those described by Agassiz as characteristic of his genus 
Parypha. 
Tubularia indivisa, Linn., = Tubularia calamaris, Pallas. 
Tubularia Couthoyi, Agass. 
Tubularia coronata, Abild., = ? Tubularia gracilis, Harvey, = 
Thamnocnidia coronata, Agass. 
Tubularia spectabilis, Agass. (sp.), = Thamnocnidia spectabilis, 
Agass. : 
' Tubularia tenella, Agass. (sp.), = Thamnocnidia tenella, Agass. 
Tubularia cristata, M‘Crady,= Parypha cristata, Agass. 
Tubularia crocea, Agass. (sp.),=Parypha crocea, Agass. 
Tubularia bellis, Allm. 
Tubularia attenuata, Allm. 
Tubularia larynx, Ellis & Soland. 
Tubularia simplex, Alder. 
?Tubularia calamaris, Van Ben. 
