MYOCHA M A . 



Plate I. 



Genus MYOCHAMA, Stutchbury. 



Testa inaquivalvis, adherens, valde irregularis, valvd dextrd 

 affixd, sinistra libera, ventricosd, ambabus dentibus duo- 

 bus marginalibus divergentibns, foveold trigond inter- 

 media ligamentnm, appendice testaced convexum, acci- 

 piente. 



Shell inequivalve, adhering, very irregular, right valve af- 

 fixed, left valve free, ventricose, both valves with two 

 marginal divergent teeth, having an intermediate pit 

 receiving the ligament, in connection with a shelly 

 appendage. 



Myochama is a parasitic ossicle-hinged bivalve peculiar 

 to Australia, and it is mostly found adhering to other bi- 

 valves. The specimens selected for illustration are at- 

 tached to shells of Pectunculus, Crassatella, and Trigonia, 

 and on two of them are two different species of Myochama, 

 the same two species on each. The hinge of Myochama is 

 composed of two divergent marginal teeth, of somewhat 

 rude construction, enclosing between them a pit for the re- 

 ception of an internal ligament, which, like the Anathia 

 tribe, has a small testaceous appendage or ossicle in con- 

 nection with it. The right valve is the adhering valve, the 

 left is more or less ventricose ; and according to the irregu- 

 larity of the surface of its place of attachment, so does it 

 more or less participate in that irregularity, apart from its 

 natural sculpture. The specimens of Myochama Anomio- 

 ides on the Pectunculus at Fig. 4 b, and on the Trigonia at 

 Fig. 4 c, are finely ridged alike in radiations from the umbo j 

 but the diagonal ribs vary, it will be seen, in bulk, accord- 

 ing to the bulk of the ribs of the shell on which each spe- 

 cimen has grown. 



Species 1. (Fig. a, Mus. Sowerby ; Fig. b, 

 Mus. Cuming.) 



Myochama Strangei. My oc. testa ovaid vel transverse 

 oblongd, tumide dichotome corrugato-liratd, plerumque 

 plus minus malleatd ; sordide luted vel earned. 



Strange's Myochama. Shell ovate or transversely ob- 

 long, tumidly dichotomously wrinkle-ridged, generally 

 more or less malleated ; dull yellow or flesh. 



Adams, Pro. Zool. Soc. 1850, p. 23. pi. 8. f. 2. 

 Myochama transversa, Adams. 



Hah. Port Jackson, Australia ; Strange. Cape Upstart, 

 Australia ; Jukes. 



This species varies in form from ovate to transversely 

 oblong, and it is extremely irregular in its growth. It is 

 of a dull colour, compared with the other species of the 

 genus, and is generally more or less decussately mal- 

 leated. 



Species 2. (Mus. Cuming.) 



Myochama Keppeliana. Myoc. tesld subquadralo- 

 ovatd, ventricosd, circa umbones tumide products, pos- 

 tice angulato-truncatd, radiatim costatd, costis planu- 

 lalis conftrtis irregularibus, nonnullis dichotomis, su- 

 perficialiter arcuatim squamidatis ; earned. 



Keppel's Myochama. Shell somewhat squarely ovate, 

 ventricose, tumidly produced about the umboes, pos- 

 teriorly angularly truncated, radiately ribbed, ribs 

 flattened, close-set, irregular, some dichotomous, su- 

 perficially arcuately scaled ; flesh-colour. 



Adams, Pro. Zool. Soc. 1852, p. 90. pi. 15. f. 1. 



Hab. Bass's Straits, Australia (in deep water) ; Keppel. 



The specimen figured of this species attached to a Pec- 

 tunculus, collected at Bass's Straits, Australia, by the Hon. 

 Captain Keppel, is, I believe, the only one known ; but it 

 appears to be well distinguished in form and sculpture 

 from its nearest ally M. Anomioides. 



Species 3. (Fig. 3, left specimen ; Fig. 4 b, upper 

 specimen.) 



Myochama tabida. Myoc. testa trigond, postice snb- 

 truncatd, radiatim liratd, liris tenuibus, numerosis, 

 conferlis, nonnullis dichotomis; sordide alba. 



The consumptive Myochama. Shell triangular, pos- 

 teriorly a little truncated, radiately ridged, ridges fine, 

 numerous, close-set, some dichotomous ; dull-white. 



Hab. Australia ; Strauge. 



This appears to me to be a well-marked species. Mr. 

 Cuming possesses several specimens, all characterized alike 

 by a triangular outline of form, by a more numerous radi- 

 ation of fine close-set ridges, and by a uniform dull-white, 

 though perfectly fresh, appearance. On the specimens of 

 Crassatella and Pectunculus, represented at Fig. 3 and 

 Fig. 4 b, M. tabidus occurs in company with M. Anomio- 

 ides. 



September, 1860. 



